Clarify default-list eligibility for internal-access plugin sources #86

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.