Add a header to the README file.

Summary

Changes

Test plan

• Tested locally with docker compose up -d --build
• Backend logs show no errors
• Relevant pytest tests pass

API documentation (if routes/schemas changed)

• Regenerated docs/api/openapi.yaml (cd backend_api_python && python scripts/export_openapi.py)
• Updated docs/agent/agent-openapi.json if /api/agent/v1 routes changed
• Breaking API changes called out below (oasdiff will fail CI otherwise)

Screenshots (if UI change)