ci: deploy FrontierChallenge site to GitHub Pages

[Chord-Chen-30]

Summary

• add a repository-root GitHub Pages workflow for the existing FrontierChallenge static site
• publish only benchmarks/frontierchallenge/site/ at /FrontierAgent/benchmarks/FrontierChallenge/
• validate the source before packaging and leave all other repository content out of the Pages artifact

Security

• pull requests build and validate but never deploy
• deployment is restricted to main pushes or manual runs from main
• job permissions are scoped to the minimum required
• GitHub-maintained actions are pinned to immutable commit SHAs
• symbolic links and root-absolute asset paths fail validation

Validation

• workflow YAML parses successfully
• local staging produced the expected nested index and 50 static files
• no symbolic links or root-absolute HTML asset references were found

[LiangcaiSu]

Support online deployment for FrontierChallenge Benchmark.