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/tools/background-task/create-background-output.ts

@@ -92,7 +92,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
           return await formatTaskResult(task, client)
         }
 
-        if (task.status === "error" || task.status === "cancelled") {
+        if (task.status === "error" || task.status === "cancelled" || task.status === "interrupt") {
           return formatTaskStatus(task)
         }
 
@@ -113,9 +113,9 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
             return await formatTaskResult(currentTask, client)
           }
 
-          if (currentTask.status === "error" || currentTask.status === "cancelled") {
-            return formatTaskStatus(currentTask)
-          }
+           if (currentTask.status === "error" || currentTask.status === "cancelled" || currentTask.status === "interrupt") {
+             return formatTaskStatus(currentTask)
+           }
         }
 
         const finalTask = manager.getTask(args.task_id)