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/Cargo.toml

@@ -18,10 +18,14 @@ tokio.workspace = true
 axum.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+reqwest.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 anyhow.workspace = true
+async-trait.workspace = true
 clap.workspace = true
+figment.workspace = true
+toml.workspace = true
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["test-util"] }