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/list-worktrees.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+set -euo pipefail
+
+# List all active git worktrees
+#
+# Usage:
+#   ./list-worktrees.sh
+
+REPO_PATH=$(git rev-parse --show-toplevel 2>/dev/null || pwd)
+cd "$REPO_PATH"
+
+echo "Active worktrees:"
+git worktree list