Feature Request: Allow user to add deferred Todos

### Describe the feature or problem you'd like to solve

If i notice something i want change, but dont want to kick the agent off track, i want to be able to add that thing as a deferred todo

### Proposed solution

Allow user to add todos to the session state `todos` table.
Introduce a command like.

```
/todo update th package XY to version Z and use the more modern element N instead of M in the codebase
```

Problem: i often look at the session progressing and then notice things a want to change. I would love to have a way to add such ideas to an "defered todo" list or something like that, so it doesn´t interrupt the current task.

首先定位会话状态中的 `todos` 表，以及处理斜杠命令的入口点。追踪当前 todo 的存储和显示方式，然后确定 `/todo` 命令应如何添加一个延后项目，同时不中断活动任务。当用户可以将请求的文本添加到会话状态，并看到它作为 todo 被保留时，即表示完成。

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