## Why
Taking a step to removing the `persistExtendedHistory` field. It's not
scalable to be persisting so much data in the rollout file and returning
it in the thread history.
When a client explicitly sends `true`, the server now tells that client
the parameter is deprecated and ignored so the caller has a clear
migration signal via the `deprecationNotice` notification.
## What changed
- Keep the `persist_extended_history` / `persistExtendedHistory` field
in the v2 protocol for compatibility, but document it as deprecated and
ignored.
- Ignore the parameter in app-server `thread/start`, `thread/resume`,
and `thread/fork`; those paths always use limited history persistence
now.
- Stop treating `persistExtendedHistory` as a running-thread resume
override mismatch.
- Emit a connection-scoped `deprecationNotice` when a request explicitly
sets `persist_extended_history: true`.
## Verification
- Added `thread_start_deprecates_persist_extended_history_true` to cover
the deprecation notice.
- `cargo test -p codex-app-server`
- `cargo test -p codex-app-server-protocol`
Owen LinandGitHub
6075b77001