fix(background-agent): address Oracle review feedback

- Remove unused storage.ts (dead code, runtime inconsistency)
- Change persist() to sync void (debounce semantics clarity)
- Add type guards in handleEvent() for event safety
- Remove unused 'pending' from BackgroundTaskStatus

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src/tools/background-task/tools.ts

@@ -207,7 +207,7 @@ Only running tasks can be cancelled.`
         task.status = "cancelled"
         task.completedAt = new Date()
 
-        await manager.persist()
+        manager.persist()
 
         return `✅ Task cancelled successfully