feat(ci): add DCO sign-off check for pull requests

[JasonXuDeveloper]

Summary

• Add GitHub Actions workflow to verify all commits have DCO sign-off
• Checks all non-merge commits in PRs targeting master
• Provides clear error messages with fix instructions

How It Works

The workflow runs on every PR and checks each commit for a Signed-off-by: line. If any commit is missing the sign-off, the check fails with instructions on how to fix it.

Example output when commits are missing sign-off:

❌ abc1234: Fix some bug
   Author: Developer <dev@example.com>
   Missing 'Signed-off-by' line

To fix existing commits, you can:
  1. Amend the last commit: git commit --amend -s
  2. Rebase and sign all: git rebase --signoff HEAD~N

Required Follow-up Action

After merging this PR, you should enable branch protection rules to enforce the check:

1. Go to Settings → Branches → Branch protection rules
2. Click Add rule (or edit existing rule for master)
3. Set Branch name pattern to master
4. Enable Require status checks to pass before merging
5. Search and select "Verify DCO Sign-off"
6. Save changes

Test plan

• Verify workflow syntax is valid
• Test with a PR that has signed-off commits (should pass)
• Test with a PR missing sign-off (should fail with instructions)
• Set up branch protection rules after merging

🤖 Generated with Claude Code

[github-actions]

Unity Test Results

✅ EditMode: All tests passed
✅ PlayMode: All tests passed

Unity Version: 2022.3.55f1
Project Path: UnityProject

✅ All tests passed! The PR is ready for review.

View workflow run

Click here to view the full workflow run

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.