refactor(neuron): cut mistralrs/llamacpp, scaffold candle harness

Stage 1 of the candle-native pivot. Replaces the external-process
harness model (mistralrs over HTTP, llamacpp placeholder) with an
in-process Harness trait whose sole implementation is candle. The
trait keeps its shape so future engines slot in additively, but
start/stop default to no-ops and HarnessConfig drops endpoint and
systemd_unit since no harness needs external supervision.

Behaviour is unchanged on the wire: load_model returns a "not
implemented yet (Stage 2)" error and list_models is empty. The
gateway-side proxy, poller, and router are untouched.

CLAUDE.md Phase 11 (llama.cpp) and Phase 12 (mistral.rs COPR) are
marked superseded; the staged plan lives in
~/.claude/plans/create-a-more-aggressive-calm-naur.md.

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

crates/cortex-core/src/anthropic.rs

@@ -2,7 +2,7 @@
 //!
 //! These mirror the `/v1/messages` format used by the Anthropic API.
 //! The gateway accepts these, translates to OpenAI format, proxies to
-//! mistral.rs, then translates the response back.
+//! the inference backend (neuron), then translates the response back.
 
 use serde::{Deserialize, Serialize};
 use serde_json::Value;

crates/cortex-core/src/harness.rs

@@ -9,13 +9,13 @@ use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 
 /// Configuration for a harness instance on a neuron.
+///
+/// All current harnesses are in-process (candle); per-harness tuning
+/// (cache paths, device policies, etc.) lives in dedicated config
+/// blocks rather than on this struct.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct HarnessConfig {
     pub name: String,
-    /// Base URL of the harness (e.g. "http://localhost:8080" for mistral.rs).
-    pub endpoint: Option<String>,
-    /// Systemd unit name, if the harness is managed via systemd.
-    pub systemd_unit: Option<String>,
 }
 
 /// Health status of a harness process.
@@ -47,16 +47,24 @@ pub struct ModelInfo {
 }
 
 /// What an inference harness must do, from neuron's perspective.
+///
+/// All current harnesses are in-process — they share neuron's address
+/// space and lifecycle. `start`/`stop` therefore default to no-ops; a
+/// future process-supervising harness would override them.
 #[async_trait]
 pub trait Harness: Send + Sync {
-    /// Human-readable name (e.g. "mistralrs", "llamacpp", "comfyui").
+    /// Human-readable name (e.g. "candle").
     fn name(&self) -> &str;
 
-    /// Start the harness process if it is not already running.
-    async fn start(&self, config: &HarnessConfig) -> Result<()>;
+    /// Start the harness. Default no-op for in-process harnesses.
+    async fn start(&self, _config: &HarnessConfig) -> Result<()> {
+        Ok(())
+    }
 
-    /// Stop the harness process gracefully.
-    async fn stop(&self) -> Result<()>;
+    /// Stop the harness. Default no-op for in-process harnesses.
+    async fn stop(&self) -> Result<()> {
+        Ok(())
+    }
 
     /// Health check. Returns the harness process status.
     async fn health(&self) -> HarnessHealth;

crates/cortex-core/src/openai.rs

@@ -3,7 +3,7 @@
 //! These are a subset sufficient for chat completions (streaming + non-streaming).
 //! Fields not relevant to proxying are captured as `serde_json::Value` via
 //! `#[serde(flatten)]` so we forward them without needing to enumerate every
-//! extension field mistral.rs supports.
+//! extension field a backend might support.
 
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
@@ -22,7 +22,7 @@ pub struct ChatCompletionRequest {
     pub max_tokens: Option<u64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub stream: Option<bool>,
-    /// All other fields (tools, response_format, mistral.rs extensions, etc.)
+    /// All other fields (tools, response_format, backend extensions, etc.)
     #[serde(flatten)]
     pub extra: Value,
 }