feat: publish RFC 9727 api-catalog and homepage Link headers

[cblanc]

Summary

• Serve an RFC 9727 API catalog at /.well-known/api-catalog on postcodes.io and api.postcodes.io: linkset pointing at the OpenAPI spec, API docs and /ready, with Content-Type: application/linkset+json; profile="https://www.rfc-editor.org/info/rfc9727" and Link: </.well-known/api-catalog>; rel="api-catalog" (GET and HEAD).
• Add an RFC 8288 Link header to the homepage (catalog, spec, docs).
• A route rather than a static file: express.static skips dotfiles. Kept in api/config/well_known.ts with two lines in routes.ts.
• Why: agent-readiness scanners and answer engines look for these to find the spec and docs; both checks fail on postcodes.io today.

Test plan

• pnpm test (new test/well_known.integration.ts: GET/HEAD catalog headers and shape, homepage Link)
• After release: purge the Cloudflare cache for postcodes.io/ if the homepage is edge-cached, then curl -i https://postcodes.io/.well-known/api-catalog and curl -I https://postcodes.io/
• POST https://isitagentready.com/api/scan {"url":"https://postcodes.io"} passes discovery.apiCatalog and discoverability.linkHeaders

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[idealbot]

🎉 This issue has been resolved in version 20.1.0 🎉

The release is available on GitHub release

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