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/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "cortex-neuron"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+name = "cortex_neuron"
+path = "src/lib.rs"
+
+[[bin]]
+name = "cortex-neuron"
+path = "src/main.rs"
+
+[dependencies]
+cortex-core.workspace = true
+tokio.workspace = true
+axum.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+anyhow.workspace = true
+clap.workspace = true
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["test-util"] }
+reqwest.workspace = true