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marked superseded; the staged plan lives in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>

neuron.example.toml

@@ -8,9 +8,9 @@
 port = 13131
 
 # -- Harnesses ---------------------------------------------------------------
-# Each [[harnesses]] entry declares an inference engine managed by neuron.
+# Each [[harnesses]] entry declares an inference engine. Currently only
+# "candle" is supported — it runs in-process and uses huggingface/candle
+# for inference on local CUDA devices.
 
 [[harnesses]]
-name = "mistralrs"
-endpoint = "http://localhost:8080"
-systemd_unit = "mistralrs.service"
+name = "candle"