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marked superseded; the staged plan lives in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>

helexa-neuron.spec

@@ -37,8 +37,9 @@ Provides:       user(neuron)
 
 %description
 Neuron is a per-node daemon for cortex inference clusters. It discovers
-local GPU hardware via nvidia-smi, manages inference harnesses (mistral.rs,
-llama.cpp), and exposes an HTTP API for model lifecycle management.
+local GPU hardware via nvidia-smi, runs in-process inference via
+huggingface/candle, and exposes an HTTP API for model lifecycle
+management (load, unload, list, inference endpoint).
 
 %prep
 %autosetup