GitHub Copilot provider: support GitHub Enterprise Cloud OAuth host

[tveon]

Please explain the motivation behind the feature request.

I'm using GitHub Enterprise Cloud (.ghe.com) with Copilot. The GitHub Copilot provider's OAuth flow is hardcoded to github.com, so the device code flow never reaches my enterprise host. I can't authenticate goose with my existing Copilot license.

This would unlock goose for enterprise Copilot users who don't have direct API keys for other providers.

Describe the solution you'd like

Allow configuring the OAuth host for the GitHub Copilot provider, e.g. via a GITHUB_HOST environment variable or an interactive prompt during goose configure.

Describe alternatives you've considered

• Custom provider with OpenAI-compatible endpoint - doesn't work because the Copilot API uses OAuth token exchange, not static API keys.
• Using a different LLM provider - possible but requires separate API access that may not be available in enterprise environments.

Additional context

• Installed via brew install block-goose-cli on macOS

• goose configure -> Configure Providers -> GitHub Copilot opens github.com/login/device instead of <enterprise-host>/login/device

• I have verified this does not duplicate an existing feature request