feat: add CI/CD examples

[kamalsrini]

Summary

• Add copy-paste CI/CD examples for GitHub Actions, GitLab CI, Azure DevOps, Jenkins, pre-commit, and local agent usage.
• Document how examples validate SecuritySkills and reserve normalized JSON/SARIF artifact output paths.
• Add scripts/validate_ci_cd_examples.rb to parse YAML examples, verify required commands, check Jenkins artifacts, and syntax-check the local agent script.

Verification

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

Linear: UNI-19