- Create pkg/state package with State and Manager structs
- Implement SetLastChannel with atomic save using temp file + rename
- Implement SetLastChatID with same atomic save pattern
- Add GetLastChannel, GetLastChatID, and GetTimestamp getters
- Use sync.RWMutex for thread-safe concurrent access
- Add comprehensive tests for atomic save, concurrent access, and persistence
- Cleanup temp file if rename fails
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
