feat: Phase 31 Nexus Architect scaffold — autonomous 3D world generation
Implements the foundation for autonomous Nexus expansion:
- NexusArchitect tool with 6 operations (design_room, create_portal,
add_lighting, validate_scene, export_scene, get_summary)
- Security-first validation with banned pattern detection
- LLM prompt generators for Three.js code generation
- 48 comprehensive tests (100% pass)
- Complete documentation with API reference
Addresses: hermes-agent#42 (Phase 31)
Related: Burn Report #6
9f09bb3066