fix(background-agent): queue notifications for idle parent sessions

When a background task completes and the parent session is waiting for
user input, promptAsync() fails with an aborted error. Previously the
notification was silently dropped — lost forever.

Fix: queue the notification text in-memory on the BackgroundManager
when promptAsync fails with an aborted/idle error. On the user's next
message to that session, the queued notifications are injected into the
chat context before the agent sees the message.

- BackgroundManager: add pendingNotifications map + queuePendingNotification()
  and injectPendingNotificationsIntoChatMessage() methods
- background-notification hook: add chat.message handler that calls injection
- chat-message.ts: wire backgroundNotificationHook.chat.message into the
  message processing chain
- Add tests covering queue-on-abort and next-message delivery

src/hooks/background-notification/hook.ts

@@ -9,6 +9,14 @@ interface EventInput {
   event: Event
 }
 
+interface ChatMessageInput {
+  sessionID: string
+}
+
+interface ChatMessageOutput {
+  parts: Array<{ type: string; text?: string; [key: string]: unknown }>
+}
+
 /**
  * Background notification hook - handles event routing to BackgroundManager.
  *
@@ -20,7 +28,15 @@ export function createBackgroundNotificationHook(manager: BackgroundManager) {
     manager.handleEvent(event)
   }
 
+  const chatMessageHandler = async (
+    input: ChatMessageInput,
+    output: ChatMessageOutput,
+  ): Promise<void> => {
+    manager.injectPendingNotificationsIntoChatMessage(output, input.sessionID)
+  }
+
   return {
+    "chat.message": chatMessageHandler,
     event: eventHandler,
   }
 }