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.

Start by locating the session state `todos` table and the command handling entry point for slash commands. Trace how todos are currently stored and displayed, then determine how a `/todo` command should add a deferred item without interrupting the active task. Done means a user can add the requested text to session state and see it retained as a todo.

Written by the indexing model from the issue text.