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/task-poller.ts

@@ -80,7 +80,8 @@ export async function checkAndInterruptStaleTasks(args: {
     const sessionID = task.sessionID
     if (!startedAt || !sessionID) continue
 
-    const sessionIsRunning = sessionStatuses?.[sessionID]?.type === "running"
+    const sessionStatus = sessionStatuses?.[sessionID]?.type
+    const sessionIsRunning = sessionStatus !== undefined && sessionStatus !== "idle"
     const runtime = now - startedAt.getTime()
 
     if (!task.progress?.lastUpdate) {