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>

Cargo.lock

@@ -408,13 +408,16 @@ name = "cortex-neuron"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "axum",
  "clap",
  "cortex-core",
+ "figment",
  "reqwest",
  "serde",
  "serde_json",
  "tokio",
+ "toml",
  "tracing",
  "tracing-subscriber",
 ]