v0.5.0 — static OpenAPI artifact + Entra security scheme

Adds a committed, static OpenAPI document (openapi/openapi.json, openapi/openapi.yaml) generated from buildOpenApiDocument(), and an Entra-aware security scheme option. No wire-format change from v0.4.0 — schemas, types and endpoints are identical, so consumers adopt it without code changes. See CHANGELOG.md for detail.