Add per-agent mutex using promise chaining to serialize state transitions
for the same agent. This prevents the Time-of-Check-Time-of-Use race
condition where two concurrent requests could both read the current state,
both validate it as valid for transition, and both write, causing one to
overwrite the other's transition.
The mutex uses a Map<string, Promise<void>> with promise chaining so that:
- Concurrent transitions to the same agent are queued and executed sequentially
- Different agents can still transition concurrently without contention
- The lock is always released even if the transition throws an error
Co-Authored-By: Claude Opus 4.6 <[email protected]>
