feat: add neuron daemon with GPU discovery and health endpoints
Replace cortex-agent stub with neuron (cortex-neuron binary).
cortex-core additions:
- discovery.rs: DeviceInfo, DiscoveryResponse, DeviceHealth, HealthResponse
- harness.rs: Harness async trait, HarnessConfig, ModelSpec, ModelInfo
neuron crate (crates/neuron/):
- discovery.rs: nvidia-smi CSV parsing (pure functions) + system
discovery via uname/nvidia-smi/nvcc
- health.rs: cached GPU health polling every 5s
- api.rs: GET /discovery and GET /health axum handlers
- main.rs: CLI entrypoint with --port flag (default 9090)
- harness stubs for mistralrs (Phase 8) and llamacpp (Phase 11)
12 new tests (9 unit + 3 integration), 35 total.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>