Enhance provider override to support baseUrl-only mode

Builds on #406 to support simpler proxy use case:
- Override just baseUrl to route built-in provider through proxy
- All built-in models preserved, no need to redefine them
- Full replacement still works when models array is provided

packages/coding-agent/CHANGELOG.md

@@ -25,6 +25,7 @@
 
 ### Added
 
+- Built-in provider overrides in `models.json`: override just `baseUrl` to route a built-in provider through a proxy while keeping all its models, or define `models` to fully replace the provider ([#406](https://github.com/badlogic/pi-mono/pull/406) by [@yevhen](https://github.com/yevhen))
 - Automatic image resizing: images larger than 2000x2000 are resized for better model compatibility. Original dimensions are injected into the prompt. Controlled via `/settings` or `images.autoResize` in settings.json. ([#402](https://github.com/badlogic/pi-mono/pull/402) by [@mitsuhiko](https://github.com/mitsuhiko))
 - Alt+Enter keybind to queue follow-up messages while agent is streaming
 - `Theme` and `ThemeColor` types now exported for hooks using `ctx.ui.custom()`