Agent Skills: ring:designing-api-contracts

Designing the API contract as a real OpenAPI 3.1 specification (openapi.yaml with full paths, operations, schemas, components, Lerian error envelope, and auth schemes) from the validated TRD. Gate 4 of ring:planning-large-features, Large Track only; runs after ring:writing-trds, before ring:designing-data-model. Use when a system exposes APIs that components or clients consume. Skip for Small Track, a system with no API surface, or an unvalidated TRD.