feat(background-agent): handle "interrupt" in notifications, output, and formatting

Update notification systems to display INTERRUPTED status.

Add interrupt handling to background_output tool (terminal status).

Add interrupt-specific status note to formatTaskStatus.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/background-task-notification-template.ts

@@ -1,6 +1,6 @@
 import type { BackgroundTask } from "./types"
 
-export type BackgroundTaskNotificationStatus = "COMPLETED" | "CANCELLED"
+export type BackgroundTaskNotificationStatus = "COMPLETED" | "CANCELLED" | "INTERRUPTED"
 
 export function buildBackgroundTaskNotificationText(input: {
   task: BackgroundTask

src/features/background-agent/notification-builder.ts

@@ -9,7 +9,7 @@ export function buildBackgroundTaskNotificationText(args: {
 }): string {
   const { task, duration, allComplete, remainingCount, completedTasks } = args
   const statusText =
-    task.status === "completed" ? "COMPLETED" : task.status === "error" ? "ERROR" : "CANCELLED"
+    task.status === "completed" ? "COMPLETED" : task.status === "interrupt" ? "INTERRUPTED" : task.status === "error" ? "ERROR" : "CANCELLED"
   const errorInfo = task.error ? `\n**Error:** ${task.error}` : ""
 
   if (allComplete) {