Agent Skills: ring:designing-data-model

Designing the physical data model as a real stack-native schema (schema.sql with CREATE TABLE DDL, indexes, and constraints for Postgres/Go; schema.prisma for Prisma/TS; Postgres schema.sql as fallback) from the Gate 4 OpenAPI spec and TRD. Gate 5 of ring:planning-large-features; runs after ring:designing-api-contracts, before ring:pinning-dependency-versions. Use when the system stores persistent data. Skip for Small Track, no persistent data, or an unvalidated API contract.