fix(background-agent): use correct OpenCode session status for stale guard

OpenCode uses 'busy'/'retry'/'idle' session statuses, not 'running'.
The stale timeout guard checked for type === 'running' which never
matched, leaving all background tasks vulnerable to stale-kill even
when their sessions were actively processing.

Change sessionIsRunning to check type !== 'idle' instead, protecting
busy and retrying sessions from premature termination.

src/features/background-agent/manager.ts

@@ -1452,7 +1452,8 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
       const sessionID = task.sessionID
       if (!startedAt || !sessionID) continue
 
-      const sessionIsRunning = allStatuses[sessionID]?.type === "running"
+      const sessionStatus = allStatuses[sessionID]?.type
+      const sessionIsRunning = sessionStatus !== undefined && sessionStatus !== "idle"
       const runtime = now - startedAt.getTime()
 
       if (!task.progress?.lastUpdate) {