Support `/plugin marketplace add` for private repositories

[7nohe]

Describe the feature or problem you'd like to solve

The /plugin marketplace add command is a powerful tool for adding plugins, but currently, it does not support private repositories. This limitation restricts its usage in scenarios where teams prefer or are required to use private repositories to host their plugins.

Proposed solution

• Add support for the /plugin marketplace add command to work with private repositories.
• Provide clear documentation or guidelines for using this feature, including any prerequisites or authentication requirements.

Example prompts or workflows

No response

Additional context

In comparison, this feature is supported in Claude Code, allowing teams to work seamlessly with private repositories when adding plugins. Introducing similar support in the GitHub Copilot CLI would greatly enhance its value and usability.

[JasonEtco]

Hello, thank you for the issue! This should already be supported - adding a plugin ultimately goes through git authentication, so if you're able to access the relevant repository (public or private), that's all the matters.

Re: documentation - we have a lot of work to do to bring our docs here up to snuff, we're working on it 💅

I'm going to close this issue as I'm able to /plugin marketplace add with a private repository myself - if you try it out and it still doesn't work, and you're sure that you're able to access that repository (example, via gh api /repos/:owner/:repo), feel free to reopen and we'll investigate!