Selected component engine (#1562)

<!-- This is an auto-generated description by cubic. -->

## Summary by cubic
Enable the Dyad Engine to prioritize a user-selected component by
flagging the file as focused and sending codebase files per request.
Keeps full context when the engine is on; falls back to path-scoped
context when it’s off.

- **New Features**
- Mark the selected component file as focused when the engine is
enabled.
- Send codebase files to the engine via dyadFiles, applied to
dyad_options unless disabled.
- Maintain full chatContext with engine; restrict to the selected file
path only when engine is off.

- **Refactors**
- Removed files from getModelClient and provider APIs; file transport
moved into request payload.
- Stream handlers now pass files to model calls and include
dyadDisableFiles/dyadFiles in the request.
  - Added focused flag to CodebaseFile.

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Sends codebase files per request to the Dyad Engine, focuses the
selected component when smart context is enabled, and refactors model
client/provider APIs to remove file parameters.
> 
> - **Engine integration**:
> - Send codebase files per request via `dyadFiles` in provider options;
propagate through `simpleStreamText` and `dyad-engine` options.
> - Add `isSmartContextEnabled` from `get_model_client` and gate context
behavior accordingly.
> - **Selected component focus**:
> - When smart context is on and a component is selected, mark its file
as `focused` in `CodebaseFile` and avoid broad smart context includes;
allow on-demand reads.
> - When smart context is off, restrict `chatContext` to the selected
file path.
> - **Refactors**:
> - Remove `files` parameter from `getModelClient` and Dyad provider;
move file transport into request body.
> - Update `llm_engine_provider` to read `dyadFiles` from request and
populate `dyad_options.files` unless `dyadDisableFiles`.
> - Extend `CodebaseFile` with optional `focused` flag; thread `files`
through `chat_stream_handlers` calls.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
022b26d0197ab5b5d4f5b589f45bc230de36e0e5. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

src/utils/codebase.ts

@@ -406,6 +406,7 @@ ${content}
 export type CodebaseFile = {
   path: string;
   content: string;
+  focused?: boolean;
   force?: boolean;
 };