feat(background-agent): use "interrupt" status for promptAsync errors

Change promptAsync catch blocks to set status = "interrupt" instead of "error".

This distinguishes prompt errors from stale timeouts (cancelled) and TTL expirations (error).

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

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

src/features/background-agent/task-resumer.ts

@@ -121,7 +121,7 @@ export async function resumeBackgroundTask(args: {
     },
   }).catch((error) => {
     log("[background-agent] resume prompt error:", error)
-    existingTask.status = "error"
+    existingTask.status = "interrupt"
     const errorMessage = error instanceof Error ? error.message : String(error)
     existingTask.error = errorMessage
     existingTask.completedAt = new Date()