feat(background-agent): add "interrupt" to BackgroundTaskStatus type

Add interrupt as a terminal status for background tasks that fail due to promptAsync errors (e.g., prompt exceed, agent not found).

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

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

src/features/background-agent/types.ts

@@ -4,6 +4,7 @@ export type BackgroundTaskStatus =
   | "completed"
   | "error"
   | "cancelled"
+  | "interrupt"
 
 export interface TaskProgress {
   toolCalls: number