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.test.ts

@@ -19,6 +19,7 @@ function createMockHandlerArgs(overrides?: {
     },
     hooks: {
       stopContinuationGuard: null,
+      backgroundNotificationHook: null,
       keywordDetector: null,
       claudeCodeHooks: null,
       autoSlashCommand: null,
@@ -115,4 +116,30 @@ describe("createChatMessageHandler - TUI variant passthrough", () => {
     //#then - gate should still be marked as applied
     expect(args._appliedSessions).toContain("test-session")
   })
+
+  test("injects queued background notifications through chat.message hook", async () => {
+    //#given
+    const args = createMockHandlerArgs()
+    args.hooks.backgroundNotificationHook = {
+      "chat.message": async (
+        _input: { sessionID: string },
+        output: ChatMessageHandlerOutput,
+      ): Promise<void> => {
+        output.parts.push({
+          type: "text",
+          text: "<system-reminder>[BACKGROUND TASK COMPLETED]</system-reminder>",
+        })
+      },
+    }
+    const handler = createChatMessageHandler(args)
+    const input = createMockInput("hephaestus", { providerID: "openai", modelID: "gpt-5.3-codex" })
+    const output = createMockOutput()
+
+    //#when
+    await handler(input, output)
+
+    //#then
+    expect(output.parts).toHaveLength(1)
+    expect(output.parts[0].text).toContain("[BACKGROUND TASK COMPLETED]")
+  })
 })