Add cleanup-branch skill for post-PR branch cleanup #42

Overview

Add a cleanup-branch project-level skill that switches to master, pulls latest changes, and deletes the current feature branch — safe to invoke after a PR is merged.

Motivation

After merging a PR, the cleanup sequence (checkout master → pull → delete branch) is repetitive. A dedicated skill eliminates the need to remember the -D flag nuance and the master-guard safety check.

File Locations

Behaviour

1. Captures the current branch name before switching
2. Checks out master and pulls
3. Deletes the feature branch using -D (safe after remote merge)
4. Guards against deleting master itself

Verification

• Skill file exists at .claude/skills/cleanup-branch/SKILL.md
• Running /cleanup-branch on a feature branch switches to master, pulls, and deletes the branch
• Running /cleanup-branch on master does NOT delete master