test(tasks): fence the background integration test on an idle owner

The rewritten test tolerated the interleaving where a fast command
settles before the running turn's next pre-step claim. The notice is
then folded into a step whose scripted reply is final, the turn closes
with an empty next-step inbox, and the collection entries are never
reached — a real timeout, not a tolerated ordering.

The command now blocks on a sentinel the test creates only after the
agent has gone idle, so the wake is the only path that can deliver the
notice, and the test asserts exactly two turns.

Also apply the review's smaller points: key the wake budget by Agent
rather than object, register the budget-refill listener only under
wakeup delivery, pin the schema default and rejection like
reportDelivery does, record the retirement-window stranding as a Known
Limitation, and cross-link the partial supersession both ways.

packages/tasks/tool-tasks/README.zh.md

@@ -89,6 +89,7 @@ Track every background task id you start. You are notified in-session when a tas
 
 ## 已知限制与暂缓事项
 
+- **落在 driver 退休窗口内的结算仍会让通知搁浅**：在轮次循环最后一次检查 inbox 与 driver 提交 idle 相位之间，所有者读起来仍是繁忙，因此通知走注入且无人唤醒。steer 有同样的洞；堵上它属于 `agent-loop`。
 - **已花掉的唤醒预算不会随时间恢复**：只有用户撰写的输入才能补充，因此预算耗尽的无人值守 agent 要等到其他原因开启下一轮时才收走剩余通知。
 - **待领于空闲所有者的通知无法在该所有者释放后存活**：释放时的取消会清空未领取的 inbox，日志保留插入/取消这一对作为记录。
 - **流读取只有单一消费方**：独立观察者需要另一套运行时 API。