`/model` selection

Proposed syntax:

```
/model <provider> <model_name>
/model <abbreviated_model_name>
```

Where provider is a Vercel AI SDK provider such as:

- `openai`
- `anthropic`

And we can provide abbreviated model names such as:

- `opus` -> `anthropic:claude-opus-4-1`
- `sonnet` -> `anthropic:claude-sonnet-4`

For the most common choices.
