## Why
Enterprise MCP token exchange and sign-in failures can collapse into generic errors, hiding useful diagnostics such as an OAuth `invalid_scope` rejection or a cancelled sign-in.
## What changed
Introduce `EmaErrorDetail` to carry static operation messages and sanitized SDK diagnostics through token-source errors and enterprise login completion. Exchange failures can report the stage, HTTP status, and allowlisted OAuth error code, such as `IdentityProvider: HTTP 400: invalid_scope`, without retaining arbitrary provider text or raw error chains.
Preserve specific discovery, credential storage, cancellation, and account/configuration mismatch errors instead of replacing them with generic failure messages.
## Testing
Extend tests to cover `invalid_scope` diagnostics, specific sign-in completion errors, and exclusion of credentials from diagnostics even when an outer error context contains sensitive text.
GitOrigin-RevId: e9280a5db3565ea91da0810bab528da5ee088d64
## What changed
- Classify token-source failures by credential kind and exchange stage, with sign-in or administrator guidance that omits provider error text. Treat resource authorization failures separately from enterprise identity failures when deciding whether authentication is required.
- Add policy-bound request authority checks that return `RequestNotSent` when policy is updating or unavailable, while allowing admitted responses to finish. Support child authorities that isolate connection revocation and share response draining.
- Recheck account validity after acquiring the credential invalidation lock and check cancellation before deleting a rejected credential.
## Testing
Extend regression coverage for policy suspension during token renewal, completion of admitted requests, recovery with fresh authority, and transport retry classification. Expand provider-error tests to cover `unauthorized_client` and `invalid_client` while checking that credential text is not exposed.
GitOrigin-RevId: c0097adc35cb5fdffde1466e4bc2b5ec78114a4b
## Why
Coordinated MCP OAuth refresh must stop retrying a definitively rejected refresh token, including after the runtime is rebuilt.
## What changed
Remove the rejected refresh token while preserving the access token and credential record. Compare stored credentials under the appropriate locks before writing so a replacement login is preserved, and update file entries using their existing keys.
Keep rejection handling alive through caller cancellation and prevent reuse within the runtime even if persistence fails. For ordinary direct-keyring credentials shared across `CODEX_HOME` directories, retire the token only in the current runtime because locking does not span those directories. Report persistence failures without exposing backend diagnostics.
## Testing
Add coverage for reauthentication after runtime rebuild, legacy file keys, cancellation with a concurrent credential replacement, keyring scope, and persistence failures with diagnostic redaction.
GitOrigin-RevId: a6e602ca2e2893ed711d853becc45142d9575fab
## Why
Tokio runtime tracing can cause SQLite log writes to enqueue more writes and exporter activity to generate further OTLP exports.
## What changed
- Exclude `tokio` and `runtime` spans from OTLP export alongside `h2` spans.
- Limit `tokio::` and `runtime::` targets to `INFO` in the default log filter, and reject their `TRACE` events directly in the SQLite sink even without an outer filter.
- Box and pin the session-loop future before spawning it.
- Reorder tracing and Tokio test attributes, restrict the skills concurrency test's subscriber to the catalog-refresh span, and use the default log filter in the SQLite formatter comparison test.
GitOrigin-RevId: f545013a2469088d7f9890c1f46bb93eb5446d2d
## What changed
Allow the issuer `https://mcp.mercadopago.com/mcp` to use authorization endpoints on `https://auth.mercadopago.com` and token endpoints on `https://mcp.mercadopago.com` without issuer-bound callbacks. Remove the previous Figma exception.
## Testing
Update the provider-exception test to accept the Mercado Pago combination and reject lookalike issuer hosts, unrelated token hosts, and HTTP issuer and token URLs.
GitOrigin-RevId: 3838c7e4c32d6a2022f0011d6020e383035fe67f
## What changed
- Add `EmaAuthenticatedHttpClient` to cache and renew resource bearers, serialize concurrent renewals, and include authentication time in the request deadline. Confine bearers to the configured MCP endpoint and reject redirects.
- Return authentication-required errors for `401` responses without replaying MCP operations. Preserve a newer cached bearer when a delayed response rejects an older one.
- Add revocable request authority that cancels pending requests and ends response streams.
- Pin enterprise credentials to a persistent version advanced on save or deletion. Use short credential locks and invalidate rejected grants only when their record and version are still current, preserving newer logins.
- Hide keyring account identifiers from enterprise credential errors and traces while preserving authentication failure classification.
## Testing
Add tests for token expiry and renewal, endpoint confinement, redirects, request deadlines, revocation during renewal and streaming, delayed `401` responses, credential replacement and logout races, and credential error privacy.
GitOrigin-RevId: ee3e5cf7ecbba3d09e1ab29ed4fbb2a35eacc60d
## What changed
- Instrument MCP OAuth loads, saves, deletes, fallback cleanup, and refresh persistence with credential-free storage metrics, distinguishing configured policy from operations on a pinned store.
- Retain the originating client across background tasks, blocking storage operations, and session recovery, including recovery after startup found no credentials.
- Preserve typed keyring and lock error categories for metrics and remove raw error details from storage warnings.
## Testing
Add tests for policy and pinned-store outcomes, best-effort cleanup failures, wrapped lock errors, and successful or failed refresh persistence without duplicate observations. Add recovery tests that verify client attribution after anonymous startup and across threads with different originators.
GitOrigin-RevId: 5ea4528bcdcf500c8ffdf1d9c7008ea52112c726
## Why
The app server previously rejected OAuth login for enterprise-managed MCP servers. These servers need an explicit sign-in flow that ties saved authorization to the active Codex user and workspace and prevents canceled or superseded attempts from saving credentials.
## What changed
- Enable enterprise sign-in through `mcpServer/oauth/login` for configured servers with a connected thread, trusted IdP registration, and `features.use_xaa` enabled. Scope shared enterprise credentials by user, workspace, issuer, and client ID.
- Add `loginId` to MCP OAuth login responses and completion notifications, preserving compatibility with older servers that omit it.
- Support enterprise login replacement and `account/login/cancel`, wait for callback listener shutdown before replacement, and recheck account and configuration before saving grants. Fence stale attempts across processes and account changes.
- Remove shared enterprise authorization during account logout and `codex mcp logout`, including when the feature is disabled. Continue account logout if enterprise cleanup fails, and invalidate MCP runtimes on account changes.
Start a fresh session after successful enterprise sign-in to use the saved grant.
## Testing
Add regression coverage for login replacement and cancellation, callback port reuse, account changes during setup, cross-process logout invalidation, persisted workspace policy selection, cleanup failures, and responses from older servers without `loginId`.
GitOrigin-RevId: 7ed4caaab4659ed1e67064262cf5cee48e3a26ac
## Why
MCP OAuth configuration supported a pre-registered client ID but could not supply a client secret for token exchange or refresh.
## What changed
- Add `oauth.client_secret` and `codex mcp add --oauth-client-secret`, requiring a nonempty secret and client ID. Persist the secret in server configuration and accept `clientSecret` in plugin configuration.
- Pass configured credentials through CLI, app-server, and plugin login flows and token refresh. Require a new login when a configured confidential client's ID differs from stored credentials.
- Redact secrets in debug output and keep them out of authorization URLs and persisted OAuth token records.
- Invalidate cached OAuth connections when the configured client ID or secret changes.
## Testing
Add coverage for configuration validation and round trips, CLI argument redaction and subsequent login, app-server login, connection invalidation, and refresh with `client_secret_basic` and `client_secret_post` in both refresh modes. Verify secret exclusion from token records and reject mismatched client IDs before contacting the provider.
GitOrigin-RevId: 85bb0e410e9e68e6cd68eb6359badfc7eac78147
## What changed
Reject authorization endpoints with schemes other than `http` or `https` before client registration or authorization URL return. Document this constraint for `mcpServer/oauth/login`.
## Testing
Make existing asynchronous tests wait for cleanup: wait for `ThreadIdle` before injecting user input in the guardian cached-score test, and poll for queue consumption before asserting that dispatched items are gone.
GitOrigin-RevId: e5f6b4c5556234c3cc2aa56910486e892c011116
## Why
Refresh failures could leave expired MCP OAuth credentials reporting ordinary errors instead of signaling that authentication is required, preventing tool calls from offering a reconnect signal.
## What changed
- Classify provider failures and timeouts as `AuthorizationRequired` when the access token has expired. Keep proactive refresh failures as ordinary errors while the token remains valid.
- Reread stored credentials after a failed refresh and adopt a valid login completed during the request, enforcing the existing issuer binding for refresh tokens. Preserve stored credentials for later retries.
- Convert authentication-required tool-call errors into a reconnect message with `mcp/www_authenticate` metadata, without exposing provider or transport details.
## Testing
Add coverage for startup and runtime OAuth recovery, proactive refresh failures, concurrent login adoption, issuer mismatches, credential preservation, and successful retries. Verify failed local refreshes do not send tool calls and server-rejected calls are not replayed.
GitOrigin-RevId: 9cf64b0452685d2085df5dc0bcecee16ffe42e47
## Why
Enterprise sign-in needs to keep browser completion separate from credential storage so callers can recheck the active account and configuration before saving a grant. Logout must also prevent an earlier sign-in from restoring credentials, including from another process sharing `CODEX_HOME`.
## What changed
- Add enterprise login APIs in `rmcp-client` that return an authorization URL and stage validated credentials for an explicit `commit_if` call. Store grants only in the keyring after rechecking caller authority under the credential lock.
- Require a registered client ID, published metadata matching the configured issuer, HTTP loopback callbacks, a refresh token, and a valid OIDC identity assertion. Request `openid` and `offline_access` with `prompt=consent`, and omit MCP resource indicators from authorization and code exchange.
- Persist a login generation under the credential lock so logout invalidates pending and staged sign-ins across processes, even when no grant is stored.
- Keep credentials and account identifiers out of enterprise error chains and logs, and avoid logging callback payloads when the receiver has closed.
## Testing
Add coverage for discovery validation, loopback callbacks, PKCE, staged keyring storage, cancellation, stale attempts, cross-process logout, and error/log privacy. Preserve ordinary MCP OAuth login without a refresh token, and adjust the terminal polling test deadline to include the minimum empty-poll wait.
GitOrigin-RevId: 2a27b9a26505a2f6fdecce8877f6e2c21e148f72
## What changed
- Honor `McpOAuthRefreshMode::Coordinated` for streamable HTTP MCP connections by letting RMCP refresh and persist credentials through the pinned credential store.
- Reread credentials while holding the refresh lock, persist refreshed tokens before retrying requests, and serialize login and logout writes with in-flight refreshes.
- Keep OAuth preparation outside MCP operation and handshake deadlines, and allow refresh persistence to finish if the initiating caller is cancelled.
## Testing
- Cover coordinated startup refresh, concurrent refreshes, `401` retries, credential mutation races, provider timeouts, and caller cancellation.
GitOrigin-RevId: 100ae788048087be23e26bb43f868b1fcebb007a
## What changed
- Add a test-only `CredentialStore` adapter that maps RMCP refreshes onto Codex's pinned OAuth credential backend.
- Require refresh transaction guards for mutations, preserve the pinned backend, and update the runtime credential snapshot only after persistence succeeds.
- Preserve scopes and token expiry metadata while rejecting credentials that no longer match the connection's client or issuer.
## Testing
- Cover guarded save and clear operations, persistence failures, pinned-store read failures, replacement and removal handling, and expiry round trips.
GitOrigin-RevId: eb1b98024322c03af94f5ab2e88c7dc4ae54893a
## What changed
- Allow MCP server names to contain `:`, `@`, `/`, and `.`, enabling names such as `npm:@modelcontextprotocol/server-sequential.thinking`.
- Preserve these names across `mcp add`, `get`, `list`, and `remove`, runtime tool namespaces, and OAuth credential lookup.
- Quote non-bare server names in generated `config.toml` recovery hints and keep similarly escaped OAuth credential names isolated.
## Testing
- Add CLI and runtime round-trip coverage for npm-style names.
- Add snapshot coverage for quoted recovery hints and a regression test for OAuth credential-name collisions.
GitOrigin-RevId: 97e9353cdbc544db4eb2377ddbfa1d2d539b1e04
## What changed
- Resolve stored enterprise IdP sessions against discovered authorization
metadata, requiring the configured issuer, public-client authentication, and
supported ID-JAG token exchange capabilities.
- Bind OIDC identity claims and MCP resource indicators to their configured
issuer, client, and server, and require reauthentication when pinned keyring
credentials are removed or replaced.
- Hold the credential lock while rereading refresh tokens, and isolate
enterprise credentials by reserved namespace and Codex home.
## Testing
- Cover metadata and claim validation, resource binding, credential replacement
and keyring failures, refresh locking, expired ID tokens, and credential
namespace isolation.
GitOrigin-RevId: edce3c6159f7d6831edf72e9608b3fc3f5823c83
## Why
Interactive MCP OAuth must not trust authorization metadata that could route an
authorization code or PKCE verifier to an unrelated token endpoint.
## What changed
- Require an advertised issuer to match the origin that served authorization
metadata.
- Validate authorization and token endpoint origins before starting both
pre-registered and dynamically registered client flows.
- Allow delegated endpoint origins when the server advertises issuer-bound
authorization responses, while retaining narrow compatibility exceptions for
existing providers.
## Testing
Add coverage for rejected untrusted metadata, accepted issuer-bound delegation,
legacy provider exceptions, and both client registration paths.
GitOrigin-RevId: 43c9f94c9f6ba2d23d7e373576cf9fa26861cfc3
## Why
An authorization server discovered for an MCP server can change. A stored refresh token must not be sent to a different issuer than the one that originally granted it.
## What changed
- Persist the discovered authorization server issuer with new OAuth credentials and preserve it across refreshes.
- Validate that issuer against a single pinned metadata snapshot before refreshing or adopting stored refresh credentials.
- Require reauthentication when refresh credentials have no issuer or the issuer changed. An unexpired access token can still be used without exposing its refresh token.
## Testing
Add coverage for issuer persistence, missing and changed issuers, metadata changes during startup, and refresh-token preservation.
GitOrigin-RevId: 07e48ba892f627428a28bbed5cca4ad703e2a5fd
## Why
Headers configured for an MCP resource can contain credentials and must not be
forwarded to a different origin during OAuth discovery, registration, or token
exchange.
## What changed
- Apply configured resource headers only to OAuth requests on the MCP resource
origin, while allowing OAuth-generated headers to take precedence.
- Follow credentialed OAuth redirects only when each destination remains on the
resource origin. Preserve redirect method semantics, the overall timeout, the
response size limit, and a maximum of 10 redirects.
- Use the Codex MCP user agent for requests to authorization-server origins.
## Testing
Add coverage for cross-origin header isolation, same-origin dynamic client
registration redirects, OAuth header precedence, token refresh, timeout
accounting, and response size limits.
GitOrigin-RevId: 609d06e2b065e4aa9ca5382b4ae60455f9e26db9
## What changed
- Cover non-blocking credential probes when the file or secrets store is locked, including retaining only a matching prior credential snapshot and recovering after the lock is released.
- Extend the streamable HTTP OAuth round trip to verify that user turns continue during store contention, newly discovered servers recover after contention, and logged-out servers do not reuse authenticated connections.
- Cover reconciliation of authentication failures and effective `Authorization` headers.
GitOrigin-RevId: 8e377644001bd0be04dc9f79841080094d09d199
## Why
Refreshing MCP connection identities should not stall the async executor while another process holds the OAuth credential-store lock.
## What changed
- Probe file and secrets credential stores without waiting during runtime refreshes, retaining the last known credentials when the store is contended.
- Reuse unchanged authentication failures while credentials are temporarily unavailable instead of restarting the connection.
- Run blocking credential loads on a blocking worker during MCP client construction.
- Allow OAuth fallback when a configured authorization header is missing, empty, or invalid.
GitOrigin-RevId: 38dad5606da9a1ceb30fe945000ec9151a5058fa
## Why
Concurrent MCP startup and status checks should not serialize when they only read the shared credential store. Repeated reads of the encrypted MCP OAuth store also needlessly decrypted unchanged contents.
## What changed
- Use shared locks for `File` and `Secrets` credential reads while keeping saves and deletes exclusive.
- Cache decrypted MCP OAuth secrets by store path, ciphertext, and passphrase, and invalidate the cache after writes.
## Testing
- Cover concurrent readers, reader/writer exclusion, shared credential loads, and cache invalidation after updates and deletes.
GitOrigin-RevId: f13512e6404d4919879ba5ba77a3e34e52b35640
## What changed
- Require callers to provide an HTTP client for MCP OAuth discovery and login, removing the separate direct `reqwest` path.
- Use the shared `http` and `url` types throughout the MCP client and drop its direct `reqwest` dependency.
- Preserve configured MCP headers when routing OAuth discovery through the provided client.
## Testing
- Add coverage that routed OAuth discovery forwards configured headers.
GitOrigin-RevId: 1345f56bd794626533133447597a5908e57b9b48
## What changed
- Update the `rmcp` model and transport integrations for `3.0.0-beta.3`.
- Preserve legacy `elicitation/create` form requests, schema defaults, metadata, and wire-format compatibility while adopting the new elicitation types.
- Harden OAuth discovery by using GET-first discovery without starting an MCP session, preventing cross-origin header redirects, validating authorization-server and callback issuers, and retaining compatibility with metadata that omits an issuer.
- Keep stored OAuth credentials readable and avoid requiring reauthorization after transient refresh failures.
## Testing
- Add coverage for legacy elicitation round trips, OAuth discovery variants, callback issuer validation, credential migration, and refresh failures.
GitOrigin-RevId: 756197d26f3fd347c28c400228f6b3d06ed493b7
## Why
Refreshing MCP runtime state should not restart an unchanged, ready server or
relist its tools.
## What changed
- Reconcile refreshed MCP configuration against the published connection set
and reuse connections whose transport, environment, authentication, OAuth
credentials, and client capabilities are unchanged.
- Keep tool filters, timeouts, metadata, and plugin provenance in the published
server view so those settings can change without reconnecting.
- Update elicitation authority in place for reused connections, and reconnect
when connection-defining inputs or live OAuth credentials change.
## Testing
Added coverage for unchanged-server reuse, view-only and elicitation-policy
updates, connection identity changes, OAuth credential changes, and avoiding
redundant tool listing.
GitOrigin-RevId: f04f5db5fbdde127b6a14f6aa9673112c0b557e7
## What changed
- Extend the lock-contention test helper with a callback that runs while the
aggregate store lock is held.
- Use a scoped worker thread and return its result directly after releasing the
lock.
- Reuse the helper for concurrent update tests covering both file and secrets
credential stores.
GitOrigin-RevId: 74b8b5370456e90ecd39fc2af2f0853594ca4f60
## Why
Concurrent Codex processes can otherwise refresh the same rotating token, and a
cancelled or partially persisted refresh can leave durable and in-memory MCP
credentials out of sync.
## What changed
- Serialize each credential's read-refresh-write transaction across processes,
reread the authoritative store after locking, and adopt credentials refreshed
by another process.
- Keep refresh persistence running after caller cancellation, bound lock and
provider waits independently, and preserve omitted refresh tokens and scopes.
- Fail MCP startup and operations when refresh or persistence fails instead of
continuing with stale credentials, while requiring reauthorization for
missing, unusable, or rejected refresh tokens.
- Exclude OAuth refresh time from the MCP initialization timeout.
## Testing
Add coverage for lock contention, concurrent refreshes, rejected and missing
credentials, storage failures, caller cancellation, and provider timeouts.
GitOrigin-RevId: 4d29b879bec646d2ceb922b526dc793b1a1f5423
[Codex Thread
019edd6d-6f14-74e2-853c-345d1803d4a6](https://codex-thread-link.openai.chatgpt-team.site/thread/019edd6d-6f14-74e2-853c-345d1803d4a6)
## Stack
Review and merge in order. Every layer is independently correct and
documents its safe stopping point.
1. [openai/codex#30292](https://github.com/openai/codex/pull/30292) —
aggregate File/Secrets store locking
2. [openai/codex#30293](https://github.com/openai/codex/pull/30293) —
resolve and lifecycle-pin the exact OAuth store
3. [openai/codex#30416](https://github.com/openai/codex/pull/30416) —
serialized authoritative refresh transaction
4. [openai/codex#30294](https://github.com/openai/codex/pull/30294) —
Codex-owned transport refresh and one-shot 401 recovery
5. [openai/codex#30295](https://github.com/openai/codex/pull/30295) —
login/logout transaction serialization
6. [openai/codex#30296](https://github.com/openai/codex/pull/30296) —
diagnostic-only Auto store drift reporting
**This PR is layer 2.**
## Why
`Auto` is keyring-first with a File fallback, but re-evaluating that
policy during transport reconstruction or persistence can make one MCP
client read from one store and later write to another. With rotating
refresh tokens, the second store may contain an older token. This layer
makes the source selected at client startup explicit and keeps that
authority stable for the client lifecycle.
## What this PR does
- Keeps `resolve_oauth_tokens_from_store_policy` as the single
configured-policy entry point and returns both credentials and the
concrete File or Keyring source that supplied them.
- Puts exact `load`, `save`, and `delete` operations on
`ResolvedOAuthCredentialStore`, making “resolve configured policy” and
“use the selected authority” distinct at call sites.
- Pins the first concrete source in `pinned_credential_store` in the
transport recipe, so initialization retries and session reconstruction
cannot re-evaluate `Auto` and adopt another store.
- Gives `OAuthPersistor` the resolved store and keeps subsequent
persistence and removal on that authority.
- Uses a typed keyring-load error to distinguish aggregate-store
coordination failures from ordinary backend failures; a coordination
failure is surfaced instead of triggering File fallback.
- Keeps login-time `Auto` behavior unchanged: prefer Keyring, fall back
to File when unavailable, and clean up legacy File state after a
successful keyring save.
- Adds structured server/backend context when fallback cleanup fails.
## Explicit decisions and non-goals
- The selection is lifecycle-local and in memory. This PR does not add a
durable backend selector, migration, reconciliation registry, or global
source of truth outside `CODEX_HOME`.
- `Auto` may choose File at the start of a later process if keyring
availability changes. Once this client resolves, a selected-store
failure is returned instead of hot-switching.
- Different `CODEX_HOME` instances remain independent even when they can
access the same Direct keyring credential.
- Cross-process refresh serialization is intentionally not part of this
layer.
## Safe stopping point
This PR can merge alone. A single MCP client no longer hot-switches
credential stores across transport rebuilds or persistence. Two
processes can still refresh the same selected credential concurrently
until layer 3.
## Review size
The net layer is 9 files, +668/−144. The production change remains
focused on store resolution and lifecycle pinning; the largest follow-up
is integration coverage that drives real session recovery.
## Validation
- `just test -p codex-rmcp-client` (99 passed; 5 expected skips)
- Real-client 404 recovery coverage with different Keyring and File
tokens; captured bearer headers prove the stale File token is never sent
- Mutation check: removing the lifecycle pin makes that integration
regression fail by observing the stale File token
[Codex Thread
019edd6d-6f14-74e2-853c-345d1803d4a6](https://codex-thread-link.openai.chatgpt-team.site/thread/019edd6d-6f14-74e2-853c-345d1803d4a6)
## Stack
Review and merge in order. Every layer is independently correct and
documents its safe stopping point.
1. [openai/codex#30292](https://github.com/openai/codex/pull/30292) —
aggregate File/Secrets store locking
2. [openai/codex#30293](https://github.com/openai/codex/pull/30293) —
resolve and lifecycle-pin the exact OAuth store
3. [openai/codex#30416](https://github.com/openai/codex/pull/30416) —
serialized authoritative refresh transaction
4. [openai/codex#30294](https://github.com/openai/codex/pull/30294) —
Codex-owned transport refresh and one-shot 401 recovery
5. [openai/codex#30295](https://github.com/openai/codex/pull/30295) —
login/logout transaction serialization
6. [openai/codex#30296](https://github.com/openai/codex/pull/30296) —
diagnostic-only Auto store drift reporting
**This PR is layer 1.**
## Why
MCP OAuth credentials stored in File or Secrets share one aggregate map.
Concurrent read-modify-write operations for different MCP servers can
both read the same snapshot and let the later write discard the earlier
update. That is a correctness problem independent of refresh-token
rotation.
## What this PR does
- Adds a bounded cross-process lock around aggregate File and Secrets
loads, saves, and deletes.
- Distinguishes aggregate-lock failures from Secrets backend
unavailability, so Auto can fall back only for the latter and cannot
bypass serialization by reading or writing File.
- Keeps Direct keyring operations outside this lock because they are
already per credential.
- Releases the Secrets aggregate lock before legacy File cleanup so
cross-store cleanup cannot create nested aggregate-lock ordering.
- Tests actual contention by waiting for an observed `WouldBlock`,
rather than assuming a sleeping worker reached the lock.
- Tests load and save with only the Secrets lock path broken while
fallback File remains readable and writable.
## Decisions and non-goals
- This lock protects aggregate-store read-modify-write integrity only.
It does not choose a credential authority or serialize an OAuth refresh
transaction.
- The lock is scoped to the active `CODEX_HOME`, matching the aggregate
files it protects.
- Lock waits are bounded, and coordination failures are surfaced rather
than treated as evidence that Secrets is unavailable.
## Safe stopping point
This PR can merge alone. It prevents lost updates and partial aggregate
reads. Auto can still resolve again during a client lifecycle until
layer 2, and concurrent refreshes remain possible until layer 3.
## Validation
- `just test -p codex-rmcp-client` (96 passed; expected environment
skips)
- Focused aggregate File/Secrets lock contention and Auto fallback tests
jif