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/harness/mod.rs

@@ -1,4 +1,105 @@
-// Harness registry. Implementations added in Phase 8+.
+//! Harness registry — maps harness names to trait implementations.
 
 pub mod llamacpp;
 pub mod mistralrs;
+
+use anyhow::Result;
+use cortex_core::harness::{Harness, HarnessConfig, ModelInfo, ModelSpec};
+use std::collections::HashMap;
+
+/// Registry of available harness implementations.
+pub struct HarnessRegistry {
+    harnesses: HashMap<String, Box<dyn Harness>>,
+}
+
+impl Default for HarnessRegistry {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl HarnessRegistry {
+    pub fn new() -> Self {
+        Self {
+            harnesses: HashMap::new(),
+        }
+    }
+
+    pub fn register(&mut self, harness: Box<dyn Harness>) {
+        self.harnesses.insert(harness.name().to_string(), harness);
+    }
+
+    /// List all registered harness names.
+    pub fn names(&self) -> Vec<String> {
+        self.harnesses.keys().cloned().collect()
+    }
+
+    /// List models from all registered harnesses.
+    pub async fn list_all_models(&self) -> Result<Vec<ModelInfo>> {
+        let mut all = Vec::new();
+        for harness in self.harnesses.values() {
+            match harness.list_models().await {
+                Ok(models) => all.extend(models),
+                Err(e) => {
+                    tracing::warn!(harness = harness.name(), error = %e, "failed to list models");
+                }
+            }
+        }
+        Ok(all)
+    }
+
+    /// Load a model on the specified harness.
+    pub async fn load_model(&self, spec: &ModelSpec) -> Result<()> {
+        let harness = self
+            .harnesses
+            .get(&spec.harness)
+            .ok_or_else(|| anyhow::anyhow!("unknown harness: {}", spec.harness))?;
+        harness.load_model(spec).await
+    }
+
+    /// Unload a model. Tries each harness until one claims it.
+    pub async fn unload_model(&self, model_id: &str) -> Result<()> {
+        for harness in self.harnesses.values() {
+            match harness.list_models().await {
+                Ok(models) if models.iter().any(|m| m.id == model_id) => {
+                    return harness.unload_model(model_id).await;
+                }
+                _ => continue,
+            }
+        }
+        anyhow::bail!("model '{model_id}' not found on any harness")
+    }
+
+    /// Get the inference endpoint for a model.
+    pub async fn inference_endpoint(&self, model_id: &str) -> Option<String> {
+        for harness in self.harnesses.values() {
+            if let Some(url) = harness.inference_endpoint(model_id).await {
+                return Some(url);
+            }
+        }
+        None
+    }
+
+    /// Build a registry from harness configs.
+    pub fn from_configs(configs: &[HarnessConfig]) -> Self {
+        let mut registry = Self::new();
+        for config in configs {
+            match config.name.as_str() {
+                "mistralrs" => {
+                    if let Some(endpoint) = &config.endpoint {
+                        registry.register(Box::new(mistralrs::MistralRsHarness::new(
+                            endpoint.clone(),
+                            config.systemd_unit.clone(),
+                        )));
+                    } else {
+                        tracing::warn!("mistralrs harness missing endpoint, skipping");
+                    }
+                }
+                other => {
+                    tracing::warn!(harness = other, "unknown harness type, skipping");
+                }
+            }
+        }
+        registry
+    }
+}