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marked superseded; the staged plan lives in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/main.rs

@@ -37,6 +37,7 @@ async fn main() -> Result<()> {
     });
 
     let port = args.port.unwrap_or(cfg.port);
+    let bind_url = format!("http://localhost:{port}");
     let start_time = Instant::now();
 
     tracing::info!("running hardware discovery");
@@ -47,8 +48,10 @@ async fn main() -> Result<()> {
         "discovery complete"
     );
 
-    // Build harness registry from config.
-    let registry = HarnessRegistry::from_configs(&cfg.harnesses);
+    // Build harness registry from config. In-process harnesses (candle)
+    // need to know neuron's own bind URL so they can return it from
+    // inference_endpoint.
+    let registry = HarnessRegistry::from_configs(&cfg.harnesses, &bind_url);
     discovery_result.harnesses = registry.names();
 
     let health_cache = Arc::new(health::HealthCache::new());