API: publish /.well-known/api-catalog (RFC 9727) #706

Description of Feature:

Publish /.well-known/api-catalog (RFC 9727): a linkset (application/linkset+json) that points at /api/v1, /api/openapi, api.docs.source.coop and /api/problems.

What value is this feature adding to Source Cooperative?

A client or agent that knows only source.coop finds the API, its machine-readable description, its human docs and its error catalogue from one well-known URL, without reading llms.txt prose.

Scope

• A route at src/app/.well-known/api-catalog/route.ts answers the linkset, and HEAD with a Link header to itself, as RFC 9727 describes.
• A test keeps it in step with public/llms.txt, so a link added to or removed from one can't be missed in the other.
• public/llms.txt gains a line for the catalog.
• API.md says where the catalog lives and what a new top-level API resource has to add to it.