- Add AgentConfig struct to validate required env vars on startup
- Change build_agent_http_client to require explicit token and URL
- Remove DEFAULT_BASE_INTERNAL_URL fallback (no more silent localhost:8000)
- Exit immediately if agent cannot connect to server on initial load
- Update integration tests to use dynamic port for BASE_INTERNAL_URL
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>