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/plugin/chat-message.ts

@@ -97,6 +97,7 @@ export function createChatMessageHandler(args: {
       setSessionModel(input.sessionID, input.model)
     }
     await hooks.stopContinuationGuard?.["chat.message"]?.(input)
+    await hooks.backgroundNotificationHook?.["chat.message"]?.(input, output)
     await hooks.runtimeFallback?.["chat.message"]?.(input, output)
     await hooks.keywordDetector?.["chat.message"]?.(input, output)
     await hooks.claudeCodeHooks?.["chat.message"]?.(input, output)