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/features/background-agent/manager.ts

@@ -93,6 +93,7 @@ export class BackgroundManager {
 
   private tasks: Map<string, BackgroundTask>
   private notifications: Map<string, BackgroundTask[]>
+  private pendingNotifications: Map<string, string[]>
   private pendingByParent: Map<string, Set<string>>  // Track pending tasks per parent for batching
   private client: OpencodeClient
   private directory: string
@@ -125,6 +126,7 @@ export class BackgroundManager {
   ) {
     this.tasks = new Map()
     this.notifications = new Map()
+    this.pendingNotifications = new Map()
     this.pendingByParent = new Map()
     this.client = ctx.client
     this.directory = ctx.directory
@@ -917,6 +919,32 @@ export class BackgroundManager {
     this.notifications.delete(sessionID)
   }
 
+  queuePendingNotification(sessionID: string | undefined, notification: string): void {
+    if (!sessionID) return
+    const existingNotifications = this.pendingNotifications.get(sessionID) ?? []
+    existingNotifications.push(notification)
+    this.pendingNotifications.set(sessionID, existingNotifications)
+  }
+
+  injectPendingNotificationsIntoChatMessage(output: { parts: Array<{ type: string; text?: string; [key: string]: unknown }> }, sessionID: string): void {
+    const pendingNotifications = this.pendingNotifications.get(sessionID)
+    if (!pendingNotifications || pendingNotifications.length === 0) {
+      return
+    }
+
+    this.pendingNotifications.delete(sessionID)
+    const notificationContent = pendingNotifications.join("\n\n")
+    const firstTextPartIndex = output.parts.findIndex((part) => part.type === "text")
+
+    if (firstTextPartIndex === -1) {
+      output.parts.unshift(createInternalAgentTextPart(notificationContent))
+      return
+    }
+
+    const originalText = output.parts[firstTextPartIndex].text ?? ""
+    output.parts[firstTextPartIndex].text = `${notificationContent}\n\n---\n\n${originalText}`
+  }
+
   /**
    * Validates that a session has actual assistant/tool output before marking complete.
    * Prevents premature completion when session.idle fires before agent responds.
@@ -1340,6 +1368,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
               taskId: task.id,
               parentSessionID: task.parentSessionID,
             })
+            this.queuePendingNotification(task.parentSessionID, notification)
           } else {
             log("[background-agent] Failed to send notification:", error)
           }
@@ -1568,6 +1597,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
     this.concurrencyManager.clear()
     this.tasks.clear()
     this.notifications.clear()
+    this.pendingNotifications.clear()
     this.pendingByParent.clear()
     this.notificationQueueByParent.clear()
     this.queuesByKey.clear()