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/session-manager.ts

@@ -191,12 +191,19 @@ export class SessionManager extends EventEmitter {
 
   /**
    * Stops all active sessions.
+   * Uses Promise.allSettled to ensure all sessions are stopped even if some fail.
    */
   async stopAllSessions(): Promise<void> {
     const stopPromises = Array.from(this.sessions.keys()).map((id) =>
       this.stopSession(id)
     );
-    await Promise.all(stopPromises);
+    const results = await Promise.allSettled(stopPromises);
+    // Log any failures but don't throw - best effort cleanup
+    for (const result of results) {
+      if (result.status === 'rejected') {
+        console.error('[SessionManager] Failed to stop session:', result.reason);
+      }
+    }
   }
 
   /**