Add framework reference registry

[kamalsrini]

Summary

• Add data/frameworks.yaml with reviewed framework references for OWASP, NIST, MITRE, CIS, CVSS, SSVC, EPSS, SLSA, CycloneDX, and SPDX.
• Add docs/framework-reference-registry.md describing registry fields, ownership, and future stale-check usage.
• Add scripts/validate_framework_registry.rb and surface the command in README/CONTRIBUTING.

Verification

• ruby scripts/validate_framework_registry.rb
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• ruby scripts/generate_quality_scorecard.rb --check
• git diff --check

Linear: UNI-21