feat: validate index metadata consistency

[kamalsrini]

Summary

• Adds a local index validator for index.yaml consistency
• Checks indexed files, missing skill/role entries, duplicate ids/files, id-to-frontmatter name, and overlapping metadata
• Updates validate-index CI to run the local validator without downloading yq
• Fixes alert-triage framework metadata mismatch between index.yaml and SKILL.md

Linear

• UNI-7

Verification

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