Adds a new REST endpoint GET /api/matrix/agents that returns the agent
registry in Matrix-compatible format. Agents from agents.yaml are served
with visual properties (color, shape, position) for 3D visualization.
Changes:
- Add matrix_router to world.py with /api/matrix/agents endpoint
- Add _build_matrix_agents_response() to transform agent config
- Add color/shape mappings for known agents with fallbacks
- Add circular position calculation for agent layout
- Register matrix_router in app.py
- Add comprehensive tests for the new endpoint
Fixes#673
