Support Dynamic Client Registration

[ChihweiLHBird]

Summary

Add support for Dynamic Client Registration so MCP clients like OpenCode can authenticate without errors.

Problem

Clients receive:

"Incompatible auth server: does not support dynamic client registration"

This blocks OpenCode and similar tools (see #2344).

Request

Implement Dynamic Client Registration (RFC 7591) for both local and remote GitHub MCP Server endpoints, or provide clear guidance and improved error handling in the meantime.

Happy to test or provide more details.