chore: add git-cleanup skill

.github/skills/git-cleanup/SKILL.md

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+---
+name: git-cleanup
+description: Clean up local git branches and remotes accumulated from PR reviews. Use when the user asks to clean branches, remove stale remotes, or tidy up the local git state.
+---
+
+# Git Cleanup
+
+Clean local branches (except `main` and current branch) and non-origin remotes.
+Useful after reviewing multiple PRs that leave behind tracking branches and contributor remotes.
+
+## Workflow
+
+### 1. Survey
+
+Run these in parallel:
+
+```bash
+git branch
+git remote
+git status --short
+git stash list
+```
+
+Present a summary of how many branches and remotes will be removed.
+
+### 2. Safety Checks — Ask Before Proceeding
+
+**Stop and ask for confirmation if any of these are true:**
+
+- Current branch is NOT `main`
+- Working tree has uncommitted changes or stashes
+- Any branch is ahead of its upstream
+
+List unmerged branches separately and let the user decide.
+
+### 3. Delete Branches
+
+```bash
+# Safe delete first (fails on unmerged branches)
+git branch | grep -v '^\*' | grep -v '^\s*main$' | xargs git branch -d 2>&1
+
+# Force-delete only with explicit user approval
+git branch -D <branch>
+```
+
+### 4. Remove Remotes
+
+```bash
+git remote | grep -v '^origin$' | xargs -I{} git remote remove {}
+```
+
+### 5. Confirm
+
+```bash
+git branch && echo "---" && git remote
+```
+
+Report what was cleaned up.
+
+## Key Rules
+
+- **NEVER** delete `main` or the current checked-out branch
+- **NEVER** force-delete unmerged branches without user confirmation
+- If in doubt, ask