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marked superseded; the staged plan lives in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/neuron/src/harness/candle.rs

@@ -0,0 +1,54 @@
+//! Candle harness — in-process inference using huggingface/candle.
+//!
+//! This is the sole `Harness` implementation. Unlike the previous
+//! mistralrs/llamacpp harnesses, candle inference runs inside the neuron
+//! process itself — no external subprocess, no systemd indirection.
+//!
+//! Stage 1 ships this as an inert skeleton; Stage 2 wires up actual
+//! model load/unload via `candle-transformers`.
+
+use anyhow::Result;
+use async_trait::async_trait;
+use cortex_core::harness::{Harness, HarnessHealth, ModelInfo, ModelSpec};
+
+pub struct CandleHarness {
+    /// URL where this neuron serves inference (its own bind address).
+    bind_url: String,
+}
+
+impl CandleHarness {
+    pub fn new(bind_url: String) -> Self {
+        Self { bind_url }
+    }
+}
+
+#[async_trait]
+impl Harness for CandleHarness {
+    fn name(&self) -> &str {
+        "candle"
+    }
+
+    async fn health(&self) -> HarnessHealth {
+        HarnessHealth {
+            name: "candle".into(),
+            running: true,
+            uptime_secs: None,
+        }
+    }
+
+    async fn list_models(&self) -> Result<Vec<ModelInfo>> {
+        Ok(Vec::new())
+    }
+
+    async fn load_model(&self, _spec: &ModelSpec) -> Result<()> {
+        anyhow::bail!("candle harness load_model not implemented yet (Stage 2)")
+    }
+
+    async fn unload_model(&self, _model_id: &str) -> Result<()> {
+        anyhow::bail!("candle harness unload_model not implemented yet (Stage 2)")
+    }
+
+    async fn inference_endpoint(&self, _model_id: &str) -> Option<String> {
+        Some(self.bind_url.clone())
+    }
+}