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/debugging/INDEX.md

@@ -1,13 +0,0 @@
-# Debugging Skills
-
-Techniques for finding and fixing bugs systematically.
-
-## Available Skills
-
-- skills/debugging/defense-in-depth - Validate at every layer data passes through to make bugs impossible. Use when invalid data causes problems deep in call stack.
-
-- skills/debugging/root-cause-tracing - Trace bugs backward through call stack to find original trigger. Use when bugs appear deep in execution and you need to find where they originate. Includes bisection script for finding test pollution.
-
-- skills/debugging/systematic-debugging - Four-phase debugging framework that ensures root cause investigation before attempting fixes. Use when encountering any technical issue, bug, or test failure. Especially critical when under time pressure or tempted to quick-fix.
-
-- skills/debugging/verification-before-completion - Never claim work is complete, tests pass, or bugs are fixed without running actual verification commands. Use before claiming complete, fixed, working, passing, clean, or ready. Before expressing satisfaction with your work. Before committing or creating PRs. When tempted to declare success.