Publish skill quality scorecard

[kamalsrini]

Summary

• Add a deterministic generated skill quality scorecard.
• Track fixture coverage, expected finding evidence counts, benign fixture counts, framework/frontmatter validation, and last audit date status.
• Add a check mode so the generated scorecard can be verified locally and in CI later.

Notes

• Accuracy and false-positive rate are explicitly marked not measured because the current harness validates static fixture evidence rather than executing skills.
• The score is a readiness score based on repository evidence, not a claim of model accuracy.

Verification

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

Linear: UNI-12