fix: memory leaks and performance optimizations

- WebServer: Store session listener refs for explicit cleanup on delete
- RespawnController: Fix clearInterval bug (was using clearTimeout)
- RespawnController: Add try/catch to interval callbacks
- PlanOrchestrator: Guarantee progressInterval cleanup with try/finally
- SubagentWatcher: Guard idle timer against deleted agents
- Session: Add threshold validation for auto-clear/compact (1k-500k)
- Session: Add token sum overflow check in restoreTokens
- Session: Use LRUMap for _recentTaskDescriptions auto-eviction

Commit from flight QR118 in an Airbus A350-1000, stable connection thanks to Starlink!

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

src/subagent-watcher.ts

@@ -1327,8 +1327,14 @@ export class SubagentWatcher extends EventEmitter {
     }
 
     const timer = setTimeout(() => {
+      // Guard against race condition: agent may have been deleted before timer fires
       const info = this.agentInfo.get(agentId);
-      if (info && info.status === 'active') {
+      if (!info) {
+        // Agent was deleted - clean up timer reference
+        this.idleTimers.delete(agentId);
+        return;
+      }
+      if (info.status === 'active') {
         info.status = 'idle';
       }
     }, IDLE_TIMEOUT_MS);