feat(www): cross-list openapi and mcp endpoint (#50180)

`/.well-known/ard.json` advertises the Management API OpenAPI spec and
the MCP server, but `/llms.txt` listed neither and
`/.well-known/api-catalog` listed only the Management API. I added both
resources to the two surfaces that were missing them, so an agent finds
the same spec and endpoint whichever discovery file it reads first.

**Changed:**
- **llms.txt gains an `## API and agent resources` section**: two
described links, the same-origin `/openapi.json` spec and
`https://mcp.supabase.com/mcp`, from a small list in
`lib/agent-resources.ts`. A named heading rather than `## Optional`,
since llmstxt.org defines Optional as links an agent may skip. The
descriptions restate ard.json's on purpose; ard.json is curated to the
ARD schema and stays untouched.
- **api-catalog lists the MCP endpoint**: added as a catalog `item` plus
its own linkset member carrying `service-doc` (the MCP guide) and
`service-meta` (the OAuth protected-resource metadata the endpoint's 401
response already points at).
- **Tests cover what the two files advertise**: `ard-catalog.test.ts`
now parses api-catalog, checks that its `item` list and its anchored
members agree, and runs every same-origin URL from api-catalog and the
llms.txt resource list through the existing dead-URL resolver (public
file, app route, rewrite, or docs guide). The www tests workflow now
checks out `apps/docs/content/guides` (the directory the llms.txt route
already reads at runtime) and runs on changes to it, so moving a guide
that a catalog links to fails that PR rather than the next www one.

**Note:** `/openapi.json` is an external rewrite served uncached on
every request (338 KB). I tried `Cache-Control` and then the documented
`x-vercel-enable-rewrite-caching` + `CDN-Cache-Control` pair on that
path; the preview kept returning `x-vercel-cache: MISS`, so both are
reverted. Caching the alias is a separate change.

## To test

Tested on Vercel preview:
- [x] `curl -s <preview>/llms.txt | tail -5`: expect an `## API and
agent resources` heading followed by the OpenAPI spec link and the MCP
server link; the diff against production `llms.txt` is those appended
lines only
- [x] `curl -s <preview>/.well-known/api-catalog | jq '.linkset[2]'`:
expect a member anchored at `https://mcp.supabase.com/mcp` with
`service-doc` and `service-meta`, served as `application/linkset+json`

## Linear
- fixes GROWTH-1207


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Added API and agent resource links to `llms.txt`, including the
Management API specification and MCP server.
- Added the Supabase MCP server to the API catalog with service
documentation and metadata links.

- **Tests**
- Expanded catalog validation to cover API catalog entries, agent
resources, and documentation guide links.
  - Updated pull request checks to run when guide content changes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/www-tests.yml

@@ -11,6 +11,9 @@ on:
       - 'apps/www/content/md/**'
       - 'apps/www/scripts/**/*.mjs'
       - 'apps/www/public/.well-known/**'
+      # www catalog tests check that linked docs guides exist, so guide changes
+      # must trigger these tests and their sources must be included in checkout.
+      - 'apps/docs/content/guides/**'
 
 # Cancel old builds on new commit for same workflow + branch/PR
 concurrency:
@@ -33,6 +36,7 @@ jobs:
           persist-credentials: false
           sparse-checkout: |
             apps/www
+            apps/docs/content/guides
             packages
             supabase
             patches