[codex] Load user instructions through an injected provider (#27101)

## Why

We want to remove implicit use of `$CODEX_HOME` from `codex-core` and
make embedders responsible for supplying user-level instructions. This
also ensures user instructions load when no primary environment is
selected.

## What changed

Stacked on #27415, which makes `codex exec` surface thread-scoped
runtime warnings.

- Added `UserInstructionsProvider` to `codex-extension-api`, with
absolute source attribution and recoverable loading warnings.
- Added `codex-home` with the filesystem-backed provider for
`AGENTS.override.md` and `AGENTS.md`, preserving precedence, fallback,
trimming, lossy UTF-8 handling, and the existing uncapped global
instruction size.
- Removed global instruction loading from `Config` and require
`ThreadManager` callers to inject a provider.
- Load provider instructions once for each fresh root runtime, including
runtimes without a primary environment. Running sessions retain their
snapshot, while child agents inherit the parent snapshot without
invoking the provider.
- Keep provider instructions separate while loading project `AGENTS.md`,
then assemble the model-visible instructions with the existing ordering,
source attribution, warning, and turn-context behavior.
- Wired the Codex home provider through the CLI, app server, MCP server,
core facade, and thread-manager sample.

## Validation

- `just test -p codex-home -p codex-extension-api`
- `just test -p codex-core agents_md`
- `just test -p codex-core guardian`
- `just test -p codex-app-server
thread_start_without_selected_environment_includes_only_global_instruction_source`
- `just test -p codex-exec warning`
- `just bazel-lock-check`

codex-rs/core/src/codex_delegate.rs

@@ -5,6 +5,7 @@ use async_channel::Receiver;
 use async_channel::Sender;
 use codex_analytics::GuardianApprovalRequestSource;
 use codex_async_utils::OrCancelExt;
+use codex_extension_api::LoadedUserInstructions;
 use codex_protocol::protocol::ApplyPatchApprovalRequestEvent;
 use codex_protocol::protocol::Event;
 use codex_protocol::protocol::EventMsg;
@@ -79,8 +80,13 @@ pub(crate) async fn run_codex_thread_interactive(
     let (tx_ops, rx_ops) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
     let conversation_history = initial_history.unwrap_or(InitialHistory::New);
     let forked_from_thread_id = conversation_history.forked_from_id();
+    let user_instructions = LoadedUserInstructions {
+        instructions: parent_session.user_instructions().await,
+        warnings: Vec::new(),
+    };
     let CodexSpawnOk { codex, .. } = Box::pin(Codex::spawn(CodexSpawnArgs {
         config,
+        user_instructions,
         installation_id: parent_session.installation_id.clone(),
         auth_manager,
         models_manager,