GET /api/v1/openapi.json answers the OpenAPI document, the same as /contract/openapi.json #4149

Seen on production: /api/v1/openapi.json, the address an agent tries first, is a 404. The document lives at /contract/openapi.json (src/http/mcp.ts).

Done:

• [ ] GET /api/v1/openapi.json answers 308 to /contract/openapi.json, cacheable.
• [ ] The here-manifest's docs, or llms.txt if that is simpler, names the OpenAPI address once.
• [ ] A test asserts the redirect and its target's 200.

Files: src/http/mcp.ts or src/http/api.ts, a test. Before pushing: bun x tsc --noEmit and the tests named here.