feat: add skill frontmatter schema validation

[kamalsrini]

Summary

• Adds a machine-readable JSON schema for SKILL.md frontmatter
• Adds a Ruby stdlib validator for all skills and role bundles
• Replaces grep-based frontmatter linting with schema validation in CI
• Fixes one invalid ISO 27001 framework value so current skills parse as YAML
• Documents the local validation command in README and CONTRIBUTING

Linear

• UNI-6

Verification

• ruby scripts/validate_skill_schema.rb
• ruby -c scripts/validate_skill_schema.rb
• ruby -rjson -e 'JSON.parse(File.read("schemas/skill.schema.json")); puts "schema json ok"'
• git diff --check