Document `/ide` support for integration with other IDEs like Visual Studio, Eclipse, ... (not only VSCode)

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

We need proper documentation of the internals of the /ide support

### Additional context

I made a VS extension (https://github.com/sailro/CopilotCliIde) by understanding how the current copilot-cli / VSCode link is working:

- discovery with lock files
- mcp server on top of http/named pipes
- tools like `get_selection`, `get_diagnostics`, `update_session_name`.
- SSE when a file is selected, or the selection changed in the IDE.

I did not find any documentation or code samples about this. Especially regarding diff support which I would like to implement (and it seems it is not working for now in both VSCode & Insiders).

Also I spotted that the selected file is displayed with a specific and hardcoded color depending on the `ideName` in the lock file:
- blue for VS Code
- cyan for VS Code Insiders
- white for others (perhaps you should add purple for Visual Studio :p)

Any pointers appreciated !
Thanks!