feat: unpublish before publish to allow same-version updates

When republishing the same version during local development, the script
now unpublishes the existing version first. If the version doesn't exist
in the registry yet, the unpublish gracefully fails and publish continues.

🤖 Generated with [Claude Code](https://claude.com/claude-code

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>