fix(background-agent): preserve parent activity across idle events

Cubic caught that clearing recent parent activity on session.idle could reopen the stale-idle race. Keep the fresh activity marker until its normal expiry so background completions still defer while the parent turn is actively reasoning.

Red: parent idle after fresh reasoning delta dispatched an unexpected parent wake. Green: focused parent-wake regression suite passes.

src/features/background-agent/parent-wake-notifier.ts

@@ -122,10 +122,6 @@ export class ParentWakeNotifier {
     this.recentParentSessionActivity.set(sessionID, Date.now())
   }
 
-  clearParentSessionActivity(sessionID: string): void {
-    this.recentParentSessionActivity.delete(sessionID)
-  }
-
   queuePendingParentWake(
     sessionID: string,
     notification: string,