OAuth authorization server metadata for the MCP resource

RFC 8414 metadata for the path-scoped issuer {base}/mcp.

RFC 8414 §3.1 path insertion: this is the metadata URL clients derive for the issuer {base}/mcp named by the protected-resource document. It advertises the anonymous OAuth-client registration door (/oauth-clients) and the public-client profile (none + PKCE S256) — the root document is a separate, unchanged surface whose registration_endpoint remains the agent /register.