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/features/background-agent/index.ts

@@ -1,3 +1,2 @@
 export * from "./types"
 export { BackgroundManager } from "./manager"
-export * from "./storage"