docs: add auto-update and SessionStart hook documentation

[a5m0]

• Add comprehensive auto-update documentation to README

  • Explain how plugin auto-updates work (at Claude Code startup)
  • Document manual update commands
  • Clarify what gets updated (skill files, metadata) vs what doesn't (Python deps)
  • Add version checking instructions
  • Include instructions for disabling auto-updates

• Add SessionStart hook for automatic setup in Claude Code on the web

  • Create scripts/setup-session.sh for automatic dependency installation
  • Add .claude/settings.json to configure SessionStart hook
  • Hook installs patchright and Chrome browser automatically
  • Idempotent design - checks existing installations before reinstalling
  • Supports both plugin and standalone skill installations

• Add CLAUDE.md repository documentation

  • Comprehensive documentation for Claude about the repository structure
  • Explains automatic setup process and SessionStart hook
  • Documents environment variables and session configuration
  • Includes troubleshooting section

• Update README with SessionStart hook documentation

  • Add "Claude Code on the Web - Automatic Setup" section
  • Update installation instructions to distinguish web (automatic) vs local (manual)
  • Add "Clone Repository" option for full SessionStart hook support
  • Update "Claude Code Web Environments" section to reference SessionStart hook
  • Note limitations of standalone skill installation (no SessionStart hook)

This makes it clear to users:

1. How plugin auto-updates work and how to control them
2. That setup is automatic in Claude Code on the web
3. What the SessionStart hook does and when it runs
4. How to choose between plugin, standalone, and repository installation