feat(ui): pick the AI model from the resolved CLI's models

[piekstra]

Summary

Settings' Model field was a free-text box with no hint of valid values. It is now a list that follows the provider:

• The options are the aliases the resolved agent CLI accepts, its default first (see docs/ai.md). Automatic offers the models of whichever CLI it found.
• Other… reveals a text field for any name the CLI understands; a saved name outside the list reopens as Other.
• The row is hidden for Off and Custom command, which receive no model.
• Switching provider resets the model, since a model name belongs to one CLI.

Evidence

Headless Chromium against lest ui on the examples, 1440x900 at 2x. A Playwright script picked an agent CLI provider, chose Sonnet, then Other with a full model name, and reloaded: the choice persisted, and the row disappeared after switching to Custom command.

Sonnet chosen; the status line reports it:

Other with a full model name:

Checks

make check passes (including a new modelChoices test). Denylist scan clean.