feat: implement mistral.rs harness and neuron model API

- MistralRsHarness: Harness trait impl wrapping mistral.rs HTTP API
  (list/load/unload models, health check, start/stop via systemd)
- HarnessRegistry: maps harness name -> Box<dyn Harness>, built from
  neuron.toml config
- Neuron API endpoints: GET /models, POST /models/load,
  POST /models/unload, GET /models/:id/endpoint
- NeuronConfig: figment-based config loading from neuron.toml
- Integration test: full model lifecycle through mock mistral.rs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/neuron/src/config.rs

@@ -0,0 +1,40 @@
+//! Neuron configuration loaded from neuron.toml.
+
+use cortex_core::harness::HarnessConfig;
+use figment::{
+    Figment,
+    providers::{Env, Format, Toml},
+};
+use serde::{Deserialize, Serialize};
+use std::path::Path;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct NeuronConfig {
+    #[serde(default = "default_port")]
+    pub port: u16,
+    #[serde(default)]
+    pub harnesses: Vec<HarnessConfig>,
+}
+
+fn default_port() -> u16 {
+    9090
+}
+
+impl NeuronConfig {
+    pub fn load(path: impl AsRef<Path>) -> Result<Self, Box<figment::Error>> {
+        Figment::new()
+            .merge(Toml::file(path))
+            .merge(Env::prefixed("NEURON_").split("__"))
+            .extract()
+            .map_err(Box::new)
+    }
+}
+
+impl Default for NeuronConfig {
+    fn default() -> Self {
+        Self {
+            port: 9090,
+            harnesses: vec![],
+        }
+    }
+}