fix: handle fork PRs in auto-format workflow

[DayuanJiang]

Summary

• Fix auto-format workflow failing for fork PRs
• Use github.event.pull_request.head.sha instead of github.head_ref for checkout (works for forks)
• For fork PRs: fail with helpful message if formatting is needed (can't push to forks)
• For same-repo PRs: auto-commit and push as before
• Simplified push logic: push directly from detached HEAD with git push origin HEAD:<branch>

Problem

The workflow was failing for fork-based PRs (like PR #413) because:

1. It tried to checkout github.head_ref (branch name) which only exists in the fork, not the main repo
2. Even if checkout worked, GITHUB_TOKEN can't push to forks

Solution

• Checkout by SHA instead of branch name (works for any PR)
• Detect fork PRs and fail gracefully with instructions for contributors
• Keep auto-format behavior for same-repo PRs