docs: make reference metadata optional

docs/requirements/README.md

@@ -77,11 +77,12 @@ Requirement Agent는 솔버 기능 요청을 검증 가능한 요구조건으로
 Expected location: `reference/<model-id>/`
 
 - `model.inp`: required | not-applicable
-- `metadata.json`: required
+- `metadata.json`: optional; if present, inspect read-only and cross-check against the approved Reference Model Contract
 - `<model-id>_displacements.csv`: required | not-applicable
 - `<model-id>_reactions.csv`: required | not-applicable
 - `<model-id>_internalforces.csv`: required | not-applicable
 - `<model-id>_stresses.csv`: required | not-applicable
+- Reference Model Contract metadata: required; record provenance, generator/version, units, coordinate system, step/frame identity, schema, tolerance, artifact inventory, and limitations
 
 ## Requirement Verification Matrix