Add CODEOWNERS review gates

[kamalsrini]

Summary

• Add explicit CODEOWNERS review gates for AppSec, Cloud, AI Security, Compliance, and SecOps skill domains.
• Add adjacent domain ownership patterns and framework registry ownership routing.
• Add scripts/validate_codeowners.rb plus a CI workflow to enforce required domain patterns.

Verification

• ruby scripts/validate_codeowners.rb
• Parsed .github/workflows/validate-codeowners.yml as YAML.
• ruby scripts/validate_framework_registry.rb --stale --max-age-days 365 --as-of 2026-06-16
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• git diff --check

Linear: UNI-24