feat: allow force push option on git-push promotion step

[aryasoni98]

Summary

Successfully implemented the force option for the git-push promotion step, allowing users to force push to any target branch. This resolves GitHub issue #5170.

Key Changes

• Added force: boolean configuration option (default: false)
• Updated core logic to respect force push setting
• Enhanced documentation with examples and warnings
• Added comprehensive test coverage
• Maintained full backward compatibility

Usage

- uses: git-push
  config:
    path: ./out
    targetBranch: main
    force: true  # Force push to overwrite remote history

Benefits

• ✅ Solves the original issue completely
• ✅ Safe opt-in with clear warnings
• ✅ Perfect for rendered manifests/generated content
• ✅ All tests pass, production-ready

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[krancour]

Thanks @aryasoni98. I made some suggestions about improving docs and tests, but overall this looks pretty good.