feat: add semantic-branch skill

[b4tchkn]

Summary

• Add semantic-branch skill to git-toolbox plugin
• Analyzes uncommitted changes or a free-text description to propose a branch name in {type}/{subject} format
• Detects project branch naming conventions from merged branch history before falling back to default format
• --dry-run flag previews the name and copies it to clipboard via pbcopy
• Slim down CLAUDE.md: remove version table (high churn, derivable from files), trim redundant prose per official best practices

Test plan

• Run /git-toolbox:semantic-branch with uncommitted changes and verify proposed name matches change type
• Run with --dry-run and verify name is copied to clipboard
• Run with a free-text description argument and verify name is generated correctly
• Verify merged branch convention detection works on a repo with consistent naming