Rename default branch from master to main

Update GitHub Actions workflow, skill files, and shell scripts
to reference 'main' instead of 'master'.

.claude/skills/update-pypi-dates/scripts/update_pypi_dates.sh

@@ -19,7 +19,7 @@ if [ $exit_code -eq 0 ]; then
         echo "📝 Changes detected, committing..."
         git add README.md
         git commit -m "Update PyPI project last updated dates"
-        git push origin master
+        git push origin main
         echo "✅ Changes pushed to remote"
     fi
 fi