fix: improve stability with memory leak fixes and atomic writes

Session Manager:
- Store event handlers for proper cleanup on session stop
- Add resetSessionManager() for test isolation

State Store:
- Use atomic write pattern (temp file + rename) to prevent corruption
- Add tokenStats to AppState interface

Subagent Watcher:
- Clean up pendingToolCalls on agent completion
- Add cleanupStaleAgents() for 24h+ old agents
- Clear all state maps on stop() for clean restart

Web Server:
- Remove listeners from transcript watcher before stopping
- Clean up respawn timers when session is deleted
- Use cleanupSession() in spawn orchestrator for proper resource cleanup

Frontend:
- Close existing EventSource before reconnecting to prevent duplicates
- Close failed connection before scheduling reconnect

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.ts

@@ -229,6 +229,8 @@ export interface AppState {
   config: AppConfig;
   /** Global statistics (cumulative across all sessions) */
   globalStats?: GlobalStats;
+  /** Daily token usage statistics */
+  tokenStats?: TokenStats;
 }
 
 // ========== Respawn Controller Types ==========