fix(background-agent): delay session error task cleanup

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

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

src/features/background-agent/manager.ts

@@ -908,13 +908,12 @@ export class BackgroundManager {
       }
 
       this.cleanupPendingByParent(task)
-      this.tasks.delete(task.id)
-      this.clearTaskHistoryWhenParentTasksGone(task.parentSessionID)
       this.clearNotificationsForTask(task.id)
       const toastManager = getTaskToastManager()
       if (toastManager) {
         toastManager.removeTask(task.id)
       }
+      this.scheduleTaskRemoval(task.id)
       if (task.sessionID) {
         SessionCategoryRegistry.remove(task.sessionID)
       }

src/features/background-agent/task-history-cleanup.test.ts

@@ -112,7 +112,7 @@ describe("task history cleanup", () => {
     managerUnderTest = undefined
   })
 
-  test("#given BackgroundManager with stale tasks for one parent #when pruneStaleTasksAndNotifications() runs #then only that parent's history is removed", () => {
+  test("#given BackgroundManager with stale tasks for one parent #when pruneStaleTasksAndNotifications() runs #then history is preserved until delayed cleanup", () => {
     // given
     const manager = createManager()
     managerUnderTest = manager
@@ -136,7 +136,7 @@ describe("task history cleanup", () => {
     pruneStaleTasksAndNotificationsForTest(manager)
 
     // then
-    expect(manager.taskHistory.getByParentSession("parent-1")).toHaveLength(0)
+    expect(manager.taskHistory.getByParentSession("parent-1")).toHaveLength(1)
     expect(manager.taskHistory.getByParentSession("parent-2")).toHaveLength(1)
   })
 })