Slash command tab completion

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

It would be nice to have tab completion for slash command params

### Proposed solution

https://github.com/user-attachments/assets/70d1077f-d426-48f1-a729-e5895f30262a

e.g. here... there is no hint for what the set of possible model names is

hitting tab seems to toggle back to the `/mcp` command for some reason, instead of providing a completion of the `/model` command I already selected

### Example prompts or workflows

_No response_

### Additional context

_No response_