Add markdown link checker to CI

What

Add an automated link checker that validates all markdown links (internal file references and external URLs) on every PR, preventing broken documentation.

Why

As the project grows (120+ markdown files across packages, examples, and docs), broken links accumulate silently. A CI check catches them before merge.

How

1. Add a GitHub Actions workflow using lychee or markdown-link-check
2. Configure it to:
   • Check all *.md files in the repository
   • Validate internal file references (relative paths)
   • Check external URLs (with retry and timeout)
   • Allow-list known flaky URLs (e.g., npm registry)
3. Run on pull_request events targeting main
4. Fix any currently broken links found during setup

Reference

• Existing workflows: .github/workflows/ (follow the permissions and pinning patterns)
• Recommended tool: lychee-action

Acceptance Criteria

• New workflow: .github/workflows/link-check.yml
• Action pinned to SHA (follow project convention)
• Runs on PRs targeting main
• All existing links pass (fix any broken ones found)
• Allow-list file for known exceptions