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>

Cargo.lock

@@ -88,6 +88,17 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
+[[package]]
+name = "async-trait"
+version = "0.1.89"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "atomic"
 version = "0.6.1"
@@ -338,19 +349,6 @@ version = "0.8.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
-[[package]]
-name = "cortex-agent"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "cortex-core",
- "reqwest",
- "serde",
- "serde_json",
- "tokio",
- "tracing",
-]
-
 [[package]]
 name = "cortex-cli"
 version = "0.1.0"
@@ -371,6 +369,7 @@ name = "cortex-core"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "chrono",
  "figment",
  "serde",
@@ -404,6 +403,22 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "cortex-neuron"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "axum",
+ "clap",
+ "cortex-core",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tracing",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "crossbeam-epoch"
 version = "0.9.18"