## What changed
- Add `codex-plugin-host` to launch a helper with a cleared environment, verify its readiness handshake, and expose framed stdin/stdout pipes with a 256 KiB frame limit and write backpressure.
- Support startup cancellation and process-group cleanup, retaining exit status and the last 16 KiB of stderr while omitting raw diagnostics from `Debug` output.
- Extend the Bazel crate macro to support test-only binaries and fixture entry points outside `src/`.
## Testing
Add process integration tests for framed communication, startup cancellation, helper and descendant cleanup, repeated close calls, and bounded diagnostics from startup and runtime failures.
GitOrigin-RevId: ed539e7899af06ee83a0d8c2d0ea8cec10f19afb
