feat: add git commit and branch helper skills (#378)

plugins/compound-engineering/skills/git-clean-gone-branches/scripts/clean-gone

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+# clean-gone: List local branches whose remote tracking branch is gone.
+# Outputs one branch name per line, or nothing if none found.
+# Uses `command git` to bypass aliases and RTK proxies.
+
+set -euo pipefail
+
+# Ensure we have current remote state
+command git fetch --prune 2>/dev/null
+
+# Find branches marked [gone] in tracking info.
+# `git branch -vv` output format:
+#   * main           abc1234 [origin/main] commit msg
+#   + feature-x      def5678 [origin/feature-x: gone] commit msg
+#     old-branch     789abcd [origin/old-branch: gone] commit msg
+#
+# The leading column can be: ' ' (normal), '*' (current), '+' (worktree).
+# We match lines containing ": gone]" to find branches whose remote is deleted.
+
+gone_branches=()
+
+while IFS= read -r line; do
+  # Skip the currently checked-out branch (marked with '*').
+  # git branch -D cannot delete the active branch, and attempting it
+  # would halt cleanup before other stale branches are processed.
+  if [[ "$line" =~ ^\* ]]; then
+    continue
+  fi
+
+  # Strip the leading marker character(s) and whitespace
+  # The branch name is the first non-whitespace token after the marker
+  branch_name=$(echo "$line" | sed 's/^[+* ]*//' | awk '{print $1}')
+
+  # Validate: skip empty, skip if it looks like a hash or flag, skip HEAD
+  if [[ -z "$branch_name" ]] || [[ "$branch_name" =~ ^[0-9a-f]{7,}$ ]] || [[ "$branch_name" == "HEAD" ]]; then
+    continue
+  fi
+
+  gone_branches+=("$branch_name")
+done < <(command git branch -vv 2>/dev/null | grep ': gone]')
+
+if [[ ${#gone_branches[@]} -eq 0 ]]; then
+  echo "__NONE__"
+  exit 0
+fi
+
+for branch in "${gone_branches[@]}"; do
+  echo "$branch"
+done