Command /model shows context window sizes per model

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

It will be good to have /model show context window sizes per model. Presently, we have to call / model and then /context to see the context window size.

### Proposed solution

Example output of `/model`:

```bash
Claude Sonnet 4.6 ✓ 1x 200K
Claude Sonnet 4.5 1x 200K
Claude Haiku 4.5 0.33x 200K
Claude Opus 4.6 3x 200K
Claude Opus 4.6 (1M context)(Internal only) (default) 6x 1M
Claude Opus 4.5 3x 200K
Claude Sonnet 4 1x 200K
Goldeneye (Internal Only) 1x 200K
GPT-5.4 1x 200K
GPT-5.3-Codex 1x 200K
GPT-5.2-Codex 1x 200K
GPT-5.2 1x 200K
GPT-5.1 1x 200K
GPT-5.4 mini 0.33x 200K
GPT-5 mini 0x 200K
GPT-4.1 0x 200K
```

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start at the `/model` command entry point and compare its current model listing with the `/context` command's context-window information. Update the model listing so each model displays its context window size as shown in the proposed output, then verify that the output remains readable and includes the expected values.

Written by the indexing model from the issue text.