fix(background-agent): track and cancel completion timers to prevent memory leaks (#1058)

Track setTimeout timers in notifyParentSession using a completionTimers Map.
Clear all timers on shutdown() and when tasks are deleted via session.deleted.
This prevents the BackgroundManager instance from being held in memory by
uncancelled timer callbacks.

Fixes #1043

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/features/background-agent/manager.ts

@@ -83,6 +83,7 @@ export class BackgroundManager {
 
   private queuesByKey: Map<string, QueueItem[]> = new Map()
   private processingKeys: Set<string> = new Set()
+  private completionTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
 
   constructor(
     ctx: PluginInput,
@@ -708,7 +709,11 @@ export class BackgroundManager {
          this.concurrencyManager.release(task.concurrencyKey)
          task.concurrencyKey = undefined
        }
-      // Clean up pendingByParent to prevent stale entries
+      const existingTimer = this.completionTimers.get(task.id)
+      if (existingTimer) {
+        clearTimeout(existingTimer)
+        this.completionTimers.delete(task.id)
+      }
       this.cleanupPendingByParent(task)
       this.tasks.delete(task.id)
       this.clearNotificationsForTask(task.id)
@@ -1073,14 +1078,15 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     }
 
     const taskId = task.id
-    setTimeout(() => {
-      // Guard: Only delete if task still exists (could have been deleted by session.deleted event)
+    const timer = setTimeout(() => {
+      this.completionTimers.delete(taskId)
       if (this.tasks.has(taskId)) {
         this.clearNotificationsForTask(taskId)
         this.tasks.delete(taskId)
         log("[background-agent] Removed completed task from memory:", taskId)
       }
     }, 5 * 60 * 1000)
+    this.completionTimers.set(taskId, timer)
   }
 
   private formatDuration(start: Date, end?: Date): string {
@@ -1375,7 +1381,11 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
       }
     }
 
-    // Then clear all state (cancels any remaining waiters)
+    for (const timer of this.completionTimers.values()) {
+      clearTimeout(timer)
+    }
+    this.completionTimers.clear()
+
     this.concurrencyManager.clear()
     this.tasks.clear()
     this.notifications.clear()