fix(background-agent): handle session.error and prevent zombie queue starts

Marks background tasks as error on session.error to release concurrency immediately, and skips/removes error tasks from queues to avoid zombie starts.

src/features/background-agent/session-task-cleanup.ts

@@ -0,0 +1,46 @@
+import { subagentSessions } from "../claude-code-session-state"
+import type { BackgroundTask } from "./types"
+
+export function cleanupTaskAfterSessionEnds(args: {
+  task: BackgroundTask
+  tasks: Map<string, BackgroundTask>
+  idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>>
+  completionTimers: Map<string, ReturnType<typeof setTimeout>>
+  cleanupPendingByParent: (task: BackgroundTask) => void
+  clearNotificationsForTask: (taskId: string) => void
+  releaseConcurrencyKey?: (key: string) => void
+}): void {
+  const {
+    task,
+    tasks,
+    idleDeferralTimers,
+    completionTimers,
+    cleanupPendingByParent,
+    clearNotificationsForTask,
+    releaseConcurrencyKey,
+  } = args
+
+  const completionTimer = completionTimers.get(task.id)
+  if (completionTimer) {
+    clearTimeout(completionTimer)
+    completionTimers.delete(task.id)
+  }
+
+  const idleTimer = idleDeferralTimers.get(task.id)
+  if (idleTimer) {
+    clearTimeout(idleTimer)
+    idleDeferralTimers.delete(task.id)
+  }
+
+  if (task.concurrencyKey && releaseConcurrencyKey) {
+    releaseConcurrencyKey(task.concurrencyKey)
+    task.concurrencyKey = undefined
+  }
+
+  cleanupPendingByParent(task)
+  clearNotificationsForTask(task.id)
+  tasks.delete(task.id)
+  if (task.sessionID) {
+    subagentSessions.delete(task.sessionID)
+  }
+}