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