fix: resolve divergent local/remote branch heads

[Jercik]

Summary

• warn when local and origin branch heads diverge and offer resolution
• update local branch ref to match origin when chosen (with an extra confirmation if it would discard local commits)
• reuse shared fetch helper and document the new behavior

Testing

• pnpm test
• pnpm lint

[github-actions]

🎉 This PR is included in version 1.2.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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