Skip already-published versions in publish workflow

Add skip-existing so re-running the release workflow for a version that
is already on PyPI is a no-op instead of an error.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: SAKMZ

.github/workflows/publish.yml

@@ -42,3 +42,5 @@ jobs:
           path: dist/
       - name: Publish to PyPI
         uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          skip-existing: true