Convert to Claude Code plugin marketplace

- Add .claude-plugin/plugin.json manifest for plugin installation
- Add .claude-plugin/marketplace.json for marketplace distribution
- Move skills from .claude/skills/ to skills/ at project root
- Update README with marketplace installation instructions
- Update CONTRIBUTING.md with new skills path
- Update validation scripts to use new skills directory

Users can now install via:
  /plugin marketplace add tjboudreaux/cc-thinking-skills
  /plugin install thinking-skills@thinking-skills-marketplace