Clarify default-list eligibility for internal-access plugin sources

We maintain D4NA, a Microsoft-owned plugin intended for internal colleagues. It already installs through its own configured plugin list. We want eligible colleagues to find the same plugin under **Customize → Plugins → Available**, without manually adding another list.

The source repository will remain internal for now. We understand that a listing does not grant repository access: installation would still require the user's existing authorized GitHub access, and the listing would need to make that restriction clear.

I reviewed the README, CONTRIBUTING guide and plugin source documentation, but did not find a stated policy for access-restricted sources.

**Can the built-in `copilot-plugins` list reference an internal GitHub repository, or should this use enterprise-managed plugin standards instead?** Please confirm the supported submission route before we propose an entry that some users cannot install.

This is an eligibility question only. No private source, credentials, or internal operational data are attached.

Start with the README, CONTRIBUTING guide, and plugin source documentation mentioned in the issue, then verify how the built-in copilot-plugins list handles access-restricted GitHub repositories. Done means the supported submission route and whether enterprise-managed plugin standards apply are confirmed, with the installation restriction stated clearly.

索引モデルが issue の本文から書いたものです。