Support separate GHE.com Copilot identity and GitHub Enterprise Server repository integration

The issue names no files, tests, or entry points. Start by locating the app's Copilot authentication and GitHub Enterprise repository-integration entry points, then review how credentials and hosts are currently coupled. Done means users can authenticate Copilot through ghe.com while separately connecting a private GHES instance for repository, pull request, issue, and review context.

An organization may use a split GitHub setup:

GitHub Copilot licenses are assigned to corporate accounts hosted on *.ghe.com.
Source code, pull requests, issues, reviews, etc. are hosted on a separate, privately hosted GitHub Enterprise Server (GHES) instance with a custom corporate URL.

The Copilot app requires authentication using the *.ghe.com account to access the Copilot license. However, there does not appear to be a way to additionally connect the app to the separate GHES instance.

As a result, users cannot benefit from the app's native GitHub integration with their actual development repositories, including pull requests, issues, reviews, and other repository context.

Ideally, Copilot authentication and repository-host authentication should be independent. A user should be able to authenticate Copilot through their *.ghe.com account while separately authenticating against a private GHES instance for repository, PR, issue, and review integration.

This would support enterprise environments where Copilot licensing is provided through GitHub Enterprise Cloud while source code remains hosted on a private GitHub Enterprise Server.