chore: sync install.sh from clawhalla repo

Updates public/install.sh to latest version with:
- Multi-instance support (auto port detection)
- Existing install detection (Update / Reinstall / Abort)
- Phase 1 bug fixes (W variable, services question, DATA_DIR)
- Phase 2 MC onboarding (gateway token in DB, provider step)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: danielgorgonha