feat(contract-validator): add gate contract versioning

- design-gate.md and data-gate.md declare gate_contract: v1
- domain-consultation.md Gate Command Reference includes Contract column
- validate_workflow_integration now checks contract version compatibility
- Tests added for match, mismatch, and missing contract scenarios

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/data-platform/commands/data-gate.md

@@ -1,5 +1,6 @@
 ---
 description: Data integrity compliance gate (pass/fail) for sprint execution
+gate_contract: v1
 arguments:
   - name: path
     description: File or directory to validate