fix: memory leaks and error handling improvements

- execution-bridge: track and cleanup retry timers to prevent memory leaks
- model-selector: add exhaustive type check for ModelTier
- ralph-tracker: fix division by zero when summary.total is 0
- respawn-controller: remove event listeners on stop to prevent memory leaks
- screen-manager: use Promise.allSettled for better error handling in stats
- session-manager: use Promise.allSettled for stopAllSessions resilience

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

src/respawn-controller.ts

@@ -1043,6 +1043,9 @@ export class RespawnController extends EventEmitter {
     this.log('Stopping respawn controller');
     this.aiChecker.cancel();
     this.planChecker.cancel();
+    // Remove event listeners from checkers to prevent memory leaks
+    this.aiChecker.removeAllListeners();
+    this.planChecker.removeAllListeners();
     this.clearTimers();
     this.stopDetectionUpdates();
     this.setState('stopped');