Add personal superpowers overlay system

Enables users to write and manage their own skills alongside core skills.

## Key Features:
- Auto-setup on first session: Creates ~/.config/superpowers/ git repo
- Two-tier skills: Personal skills shadow core skills when paths match
- Environment variable support: PERSONAL_SUPERPOWERS_DIR, XDG_CONFIG_HOME
- GitHub integration: Optional public repo creation for sharing skills
- CLI-agnostic: Works across Claude Code, Codex CLI, Gemini CLI (future)

## Changes:
- Added hooks/setup-personal-superpowers.sh - Auto-initializes personal repo
- Updated hooks/session-start.sh - Runs setup, offers GitHub repo creation
- Updated list-skills, skills-search - Search both personal and core skills
- Renamed skills/meta/creating-skills → writing-skills
- Added skills/meta/setting-up-personal-superpowers - Setup documentation
- Added skills/meta/sharing-skills - Contribution workflow
- Removed skills/meta/installing-skills - Old ~/.clank system
- Removed all INDEX.md files - Replaced by list-skills tool
- Updated README.md, getting-started - Document personal skills workflow

## Architecture:
~/.config/superpowers/skills/  # Personal (user-created, git-tracked)
${CLAUDE_PLUGIN_ROOT}/skills/  # Core (read-only, from plugin)

Search order: Personal first, core second (first match wins)

hooks/setup-personal-superpowers.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Setup script for personal superpowers directory
+# Creates personal superpowers directory with git repo for personal skills
+
+set -euo pipefail
+
+# Use PERSONAL_SUPERPOWERS_DIR if set, otherwise XDG_CONFIG_HOME/superpowers, otherwise ~/.config/superpowers
+SUPERPOWERS_DIR="${PERSONAL_SUPERPOWERS_DIR:-${XDG_CONFIG_HOME:-$HOME/.config}/superpowers}"
+SKILLS_DIR="${SUPERPOWERS_DIR}/skills"
+
+# Check if already set up
+if [[ -d "${SUPERPOWERS_DIR}/.git" ]] && [[ -d "${SKILLS_DIR}" ]]; then
+    # Already set up, nothing to do
+    exit 0
+fi
+
+# Create directory structure
+mkdir -p "${SKILLS_DIR}"
+
+# Create .gitignore
+cat > "${SUPERPOWERS_DIR}/.gitignore" <<'EOF'
+# Superpowers local data
+search-log.jsonl
+conversation-index/
+conversation-archive/
+EOF
+
+# Create README
+cat > "${SUPERPOWERS_DIR}/README.md" <<'EOF'
+# My Personal Superpowers
+
+Personal skills and techniques for Claude Code.
+
+Learn more about Superpowers: https://github.com/obra/superpowers
+EOF
+
+# Initialize git repo if not already initialized
+if [[ ! -d "${SUPERPOWERS_DIR}/.git" ]]; then
+    cd "${SUPERPOWERS_DIR}"
+    git init -q
+    git add .gitignore README.md
+    git commit -q -m "Initial commit: Personal superpowers setup"
+fi
+
+# Check for gh and recommend GitHub setup
+if command -v gh &> /dev/null; then
+    echo "github_cli_available=true"
+else
+    echo "github_cli_available=false"
+fi
+
+exit 0