Add CI pipeline for skill validation

[harrymunro]

Summary

• Adds a GitHub Actions workflow with 5 parallel jobs: markdown lint, YAML lint, link checking, spell checking, and cross-reference validation
• Runs on every push to main and all pull requests
• Includes a custom scripts/check-references.sh that validates SKILL.md directives, index→sub-file links, orphan detection, and cross-file references
• Configures markdownlint, yamllint, typos, and lychee with project-specific rules (Royal Navy terminology, relaxed line lengths, offline link checking)

Test plan

• scripts/check-references.sh passes locally with exit 0
• All config files (YAML, TOML) parse correctly
• All 5 CI jobs pass on GitHub Actions