fix(background-agent): clear pending parent on silent cancel

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

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

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

@@ -105,6 +105,7 @@ describe("BackgroundManager.cancelTask cleanup", () => {
 
     // then
     expect(cancelled).toBe(true)
+    expect(getPendingByParent(manager).get(task.parentSessionID)).toBeUndefined()
     runScheduledCleanup(manager, task.id)
     expect(manager.getTask(task.id)).toBeUndefined()
   })