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)

skills/getting-started/list-skills

@@ -1,22 +1,50 @@
 #!/usr/bin/env bash
 # list-skills - Show all available skill names without descriptions
 # For Claude to quickly see what exists before searching
+# Searches personal skills first, then core skills (personal shadows core)
 
 set -euo pipefail
 
-# Detect if running from repo or installed location
+# Detect core skills directory (repo or installed location)
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 if [[ "$SCRIPT_DIR" == *"/.claude/plugins/cache/"* ]]; then
     # Installed as plugin
-    SKILLS_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+    CORE_SKILLS_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
 else
     # Running from repo
-    SKILLS_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+    CORE_SKILLS_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
 fi
 
-# Find all SKILL.md files and extract their paths
-find "$SKILLS_DIR" -name "SKILL.md" -type f | \
-    sed "s|$SKILLS_DIR/||; s|/SKILL.md||" | \
-    sort
+# Use PERSONAL_SUPERPOWERS_DIR if set, otherwise XDG_CONFIG_HOME/superpowers, otherwise ~/.config/superpowers
+PERSONAL_SUPERPOWERS_DIR="${PERSONAL_SUPERPOWERS_DIR:-${XDG_CONFIG_HOME:-$HOME/.config}/superpowers}"
+PERSONAL_SKILLS_DIR="${PERSONAL_SUPERPOWERS_DIR}/skills"
+
+# Collect all skill paths with deduplication
+declare -A seen_skills
+all_skills=()
+
+# Personal skills first (take precedence)
+if [[ -d "$PERSONAL_SKILLS_DIR" ]]; then
+    while IFS= read -r file; do
+        skill_path="${file#$PERSONAL_SKILLS_DIR/}"
+        skill_path="${skill_path%/SKILL.md}"
+        if [[ -n "$skill_path" ]]; then
+            seen_skills["$skill_path"]=1
+            all_skills+=("$skill_path")
+        fi
+    done < <(find "$PERSONAL_SKILLS_DIR" -name "SKILL.md" -type f 2>/dev/null || true)
+fi
+
+# Core skills (only if not shadowed by personal)
+while IFS= read -r file; do
+    skill_path="${file#$CORE_SKILLS_DIR/}"
+    skill_path="${skill_path%/SKILL.md}"
+    if [[ -n "$skill_path" ]] && [[ -z "${seen_skills[$skill_path]:-}" ]]; then
+        all_skills+=("$skill_path")
+    fi
+done < <(find "$CORE_SKILLS_DIR" -name "SKILL.md" -type f 2>/dev/null || true)
+
+# Sort and output
+printf "%s\n" "${all_skills[@]}" | sort
 
 exit 0