## What changed
- Load an optional package-supplied config file as the lowest-precedence
configuration layer, so every existing layer can override its values.
- Report the layer as `packagedDefaults`, including its source path, through
config diagnostics and the app-server protocol.
- Return an error when a configured packaged defaults file is missing.
## Testing
- Cover precedence across packaged, system, user, and session layers.
- Cover the missing-file error path.
GitOrigin-RevId: 52406890f4d580eedabc90bf2a9ee3affa36fee1
## Why
Config layer provenance describes how effective configuration was
assembled, so it belongs with the config loader rather than in
app-server's serialized API types.
## What changed
- Moved `ConfigLayerSource`, `ConfigLayerMetadata`, and `ConfigLayer`
ownership into `codex-config`.
- Kept app-server's wire payloads unchanged and added explicit
conversions at the app boundary.
- Removed lower-level app-server-protocol dependencies from config
consumers.
## Stack
This is PR 3 of 6, stacked on [PR
#29721](https://github.com/openai/codex/pull/29721). Review only the
delta from `codex/split-auth-domain-types`. Next: [PR
#29723](https://github.com/openai/codex/pull/29723).
## Validation
- `codex-config` coverage passed.
- App-server config-manager and config RPC coverage passed.
jif