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.

首先定位工作階段狀態中的 `todos` 資料表，以及處理斜線指令的進入點。追蹤目前 todo 的儲存與顯示方式，然後判斷 `/todo` 指令應如何加入延後項目，同時不中斷使用中的工作。當使用者可以將要求的文字加入工作階段狀態，並看到它以 todo 的形式保留時，即表示完成。

由索引模型根據 Issue 內容生成。