fix: use default checkout ref and enable yaml ci

[G36maid]

1. Remove explicit ref: ${{ github.head_ref }} from reusable workflows. This ensures CI runs against the merge commit (default behavior) to catch integration issues.

2. Enable run-config-checks in main.yml to validate this repository's own configuration files (dogfooding).

Closes #34

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[github-actions]

🛡️ PR Quality Check Summary

⚠️ PR Title: No output received
⚠️ Branch Name: No output received
⚠️ Commit Messages: No output received
⚠️ Conflicts: No output received
⚠️ YAML Files: No output received
⚠️ JSON Files: No output received
⚠️ TOML Files: No output received

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🎉 All checks passed!

[G36maid]

Duplicate of #36