Fixes the "Generate git commit message" wand button being permanently
disabled on startup. Subscribes the Git Panel to language model registry
events so that it automatically re-renders once the asynchronous model
fetches complete.

Closes #55451.

## Suggested .rules additions

### GPUI Views and Global State Registries

* If a view (like `GitPanel`) reads from a global state registry (like
`LanguageModelRegistry`) during rendering, it must subscribe/observe
that registry's events and call `cx.notify()` to re-render when the
state updates (such as when models finish loading asynchronously).

Release Notes:

- Fixed the git commit message generation button showing as permanently
disabled when models load after startup.

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Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>