Mintlify method-string nav renders in dev but breaks mint export

Declaring OpenAPI endpoints in docs.json as a group with an openapi key plus pages entries like "GET /v1/health" renders correctly under mint dev, and both mint validate and mint broken-links pass. mint export then fails every one of them with "failed to generate /GET /v1/health (status 404)" because it treats the method string as a literal page path, and one failure aborts the whole export. Fix is a stub .mdx per operation containing only frontmatter title plus openapi: "GET /v1/health", with the spec declared once at the docs.json top level as "openapi": [path]. Bonus: the stub path becomes the URL, so pages stop being slugged from the operation summary and no longer move when someone rewords a summary.