feat: add parallel issue implementation capability with worktrees

Add complete capability set for orchestrating parallel issue implementation
with automated review cycles using git worktrees.

Components:
- worktrees skill: Git worktree patterns + bundled scripts for reliable operations
- spawn-issues skill: Event-driven orchestrator (Haiku) for parallel workflow
- issue-worker agent: Implements issues autonomously (Sonnet)
- code-reviewer agent: Reviews PRs with quality checks (Haiku, read-only)
- pr-fixer agent: Addresses review feedback automatically (Haiku)

Workflow: /spawn-issues creates worktrees → spawns workers → reviews PRs →
fixes feedback → iterates until approved → cleans up worktrees

Scripts handle error-prone worktree operations. Orchestrator uses event-driven
approach with task notifications for efficient parallel execution.

Co-Authored-By: Claude Code <noreply@anthropic.com>

skills/worktrees/scripts/cleanup-worktrees.sh

@@ -0,0 +1,56 @@
+#!/bin/bash
+set -euo pipefail
+
+# Clean up git worktrees
+#
+# Usage:
+#   ./cleanup-worktrees.sh <path>              # Remove specific worktree
+#   ./cleanup-worktrees.sh <directory>         # Remove all worktrees in directory
+#   ./cleanup-worktrees.sh --force <path>      # Force remove even if dirty
+
+FORCE=false
+if [ "$1" = "--force" ]; then
+  FORCE=true
+  shift
+fi
+
+TARGET="$1"
+REPO_PATH=$(git rev-parse --show-toplevel 2>/dev/null || pwd)
+
+cd "$REPO_PATH"
+
+remove_worktree() {
+  local worktree_path="$1"
+
+  if [ ! -d "$worktree_path" ]; then
+    return 0
+  fi
+
+  if [ "$FORCE" = true ]; then
+    git worktree remove "$worktree_path" --force 2>/dev/null || true
+  else
+    git worktree remove "$worktree_path" 2>/dev/null || true
+  fi
+}
+
+# Check if target is a directory containing multiple worktrees
+if [ -d "$TARGET" ]; then
+  # Check if it's a worktree itself or a directory of worktrees
+  if git worktree list | grep -q "$TARGET\$"; then
+    # It's a single worktree
+    remove_worktree "$TARGET"
+  else
+    # It's a directory, remove all worktrees inside
+    for worktree in "$TARGET"/*; do
+      if [ -d "$worktree" ]; then
+        remove_worktree "$worktree"
+      fi
+    done
+
+    # Try to remove the directory if empty
+    rmdir "$TARGET" 2>/dev/null || true
+  fi
+else
+  echo "Error: Path does not exist: $TARGET" >&2
+  exit 1
+fi