Add stale framework reference checks

[kamalsrini]

Summary

• Add stale-review mode to scripts/validate_framework_registry.rb.
• Add a weekly scheduled workflow to report framework references older than the configured review window.
• Document the stale check in README, CONTRIBUTING, and the framework registry docs.

Verification

• ruby scripts/validate_framework_registry.rb
• ruby scripts/validate_framework_registry.rb --stale --max-age-days 365 --as-of 2026-06-16
• ruby scripts/validate_framework_registry.rb --stale --max-age-days 365 --as-of 2028-01-01 (expected failure verified locally)
• 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-22