[codex] add MEDH NSC OpenAPI contract

[j-shilling]

What changed

• Replaced the MEDH NSC hello-world placeholder with the first NSC enrollment verification OpenAPI contract.
• Added the /enrollment-verifications operation, MESH security schemes, request/response examples, and service metadata.
• Added standalone JSON Schema draft 2020-12 request and response schemas for the NSC Name/DOB and SSN-only verification paths.
• Added a local AJV date format hook so service-level schema validation can compile the new schemas.

Why

The MEDH NSC spec needed to reflect the actual National Student Clearinghouse service contract rather than placeholder tooling content.

Impact

Consumers can now lint, bundle, inspect, and validate against the NSC request/response contract. The spec intentionally avoids references to temporary source files or internal extraction artifacts.

Validation

• pnpm --filter @fdsh-utils/spec-medh-nsc lint
• pnpm --filter @fdsh-utils/spec-medh-nsc bundle
• pnpm exec markdownlint-cli2 specs/medh/nsc/README.md
• git diff --check