## What changed
- Add optional `realtimeStartInstructions` and `realtimeEndInstructions` fields to `thread/realtime/start`.
- Apply the instructions when Codex enters and leaves realtime mode, while preserving the existing defaults when either field is omitted.
- Limit each field to 8,192 estimated tokens.
## Testing
- Cover custom entry and exit instructions across realtime state transitions.
- Cover token-limit validation for both fields.
GitOrigin-RevId: b9cfa1ff22e084d8a6b2ed56bd27d95ffdd6b6bd