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/cortex-core/src/discovery.rs

@@ -0,0 +1,43 @@
+//! Hardware discovery and health types shared between cortex and neuron.
+
+use serde::{Deserialize, Serialize};
+
+/// Information about a single GPU device discovered on a node.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DeviceInfo {
+    pub index: u32,
+    pub name: String,
+    pub vram_total_mb: u64,
+    pub compute_capability: String,
+}
+
+/// Full discovery response from a neuron endpoint.
+/// Returned by `GET /discovery`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DiscoveryResponse {
+    pub hostname: String,
+    pub os: String,
+    pub kernel: String,
+    pub cuda_version: Option<String>,
+    pub driver_version: Option<String>,
+    pub devices: Vec<DeviceInfo>,
+    pub harnesses: Vec<String>,
+}
+
+/// Runtime health metrics for a single GPU device.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DeviceHealth {
+    pub index: u32,
+    pub vram_used_mb: u64,
+    pub vram_free_mb: u64,
+    pub utilization_pct: u32,
+    pub temp_c: u32,
+}
+
+/// Runtime health response from a neuron endpoint.
+/// Returned by `GET /health`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HealthResponse {
+    pub uptime_secs: u64,
+    pub devices: Vec<DeviceHealth>,
+}