fix(neuron): surface full anyhow chain + ensure $HOME exists at start

Two fixes uncovered by the live validation against beast/benjy/quadbrat:

1. api.rs swallowed everything beyond the outermost anyhow context.
   The validation script reported '{"error":"fetch GGUF ...gguf"}' but
   the actual underlying hf-hub failure (cache dir creation, network,
   auth, etc.) was hidden. Switching every error response to
   format!("{e:#}") expands the full cause chain via anyhow's
   alternate Display format.

2. The neuron systemd unit declared the service user but never ensured
   /var/lib/neuron (its $HOME) existed. hf-hub defaults its cache to
   ~/.cache/huggingface/hub — when $HOME is absent the cache dir
   creation fails and the download aborts. Adding `StateDirectory=neuron`
   makes systemd create + chown that directory at activation; no spec
   change needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/api.rs

@@ -56,7 +56,7 @@ async fn list_models(State(state): State<Arc<NeuronState>>) -> impl IntoResponse
         Ok(models) => Json(json!(models)).into_response(),
         Err(e) => (
             StatusCode::INTERNAL_SERVER_ERROR,
-            Json(json!({"error": e.to_string()})),
+            Json(json!({"error": format!("{e:#}")})),
         )
             .into_response(),
     }
@@ -71,7 +71,7 @@ async fn load_model(
         Ok(()) => Json(json!({"status": "loaded"})).into_response(),
         Err(e) => (
             StatusCode::BAD_REQUEST,
-            Json(json!({"error": e.to_string()})),
+            Json(json!({"error": format!("{e:#}")})),
         )
             .into_response(),
     }
@@ -95,7 +95,11 @@ async fn unload_model(
     let registry = state.registry.read().await;
     match registry.unload_model(&model_id).await {
         Ok(()) => Json(json!({"status": "unloaded"})).into_response(),
-        Err(e) => (StatusCode::NOT_FOUND, Json(json!({"error": e.to_string()}))).into_response(),
+        Err(e) => (
+            StatusCode::NOT_FOUND,
+            Json(json!({"error": format!("{e:#}")})),
+        )
+            .into_response(),
     }
 }
 
@@ -151,7 +155,7 @@ async fn chat_completions(
                 .into_response(),
             Err(InferenceError::Other(e)) => (
                 StatusCode::INTERNAL_SERVER_ERROR,
-                Json(json!({"error": e.to_string()})),
+                Json(json!({"error": format!("{e:#}")})),
             )
                 .into_response(),
         }
@@ -165,7 +169,7 @@ async fn chat_completions(
                 .into_response(),
             Err(InferenceError::Other(e)) => (
                 StatusCode::INTERNAL_SERVER_ERROR,
-                Json(json!({"error": e.to_string()})),
+                Json(json!({"error": format!("{e:#}")})),
             )
                 .into_response(),
         }