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>

crates/neuron/src/api.rs

@@ -0,0 +1,30 @@
+//! HTTP API handlers for the neuron daemon.
+
+use crate::health::HealthCache;
+use axum::Router;
+use axum::extract::State;
+use axum::response::Json;
+use axum::routing::get;
+use cortex_core::discovery::{DiscoveryResponse, HealthResponse};
+use std::sync::Arc;
+
+/// Shared state for the neuron HTTP server.
+pub struct NeuronState {
+    pub discovery: DiscoveryResponse,
+    pub health_cache: Arc<HealthCache>,
+}
+
+/// Build the neuron API router.
+pub fn neuron_routes() -> Router<Arc<NeuronState>> {
+    Router::new()
+        .route("/discovery", get(discovery_handler))
+        .route("/health", get(health_handler))
+}
+
+async fn discovery_handler(State(state): State<Arc<NeuronState>>) -> Json<DiscoveryResponse> {
+    Json(state.discovery.clone())
+}
+
+async fn health_handler(State(state): State<Arc<NeuronState>>) -> Json<HealthResponse> {
+    Json(state.health_cache.snapshot().await)
+}