ci: add semantic PR title check

[james-willis]

Summary

• Adds amannn/action-semantic-pull-request@v5 workflow to enforce conventional commit prefixes on PR titles
• Adds conventional-pre-commit hook to validate commit messages locally
• Allowed types: feat, fix, chore, refactor, perf, test, ci, docs, build, style, revert

Test plan

• Open a PR with a non-conventional title and verify the check fails
• Run pre-commit install --hook-type commit-msg locally, then try committing with a bad prefix — should be rejected