docs: Add FAQ section for common questions

[meichuanyi]

Summary

Adds a comprehensive FAQ section to the README covering:

• What Helix is (self-healing runtime for autonomous agents)
• 6-stage pipeline table (Perceive → Gene)
• Gene Map explanation (SQLite + RL)
• Usage example (wrap function)
• Benchmarks table (99.9% vs 81.9%, Gene Map warm)
• Installation commands (npm + pip)
• Links to npm/PyPI/Docker
• MIT License

Checklist

• Documentation improvement
• No code changes
• Content based on existing README