refactor(background-agent): remove file persistence, use memory-only

- Remove background_tasks.json persistence (race condition with multiple instances)
- Pure memory-based task management
- Add logging for promptAsync errors
- Remove unused persist/restore methods

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src/tools/background-task/tools.ts

@@ -207,8 +207,6 @@ Only running tasks can be cancelled.`
         task.status = "cancelled"
         task.completedAt = new Date()
 
-        manager.persist()
-
         return `✅ Task cancelled successfully
 
 Task ID: ${task.id}