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marked superseded; the staged plan lives in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>

models.example.toml

@@ -6,7 +6,7 @@
 
 [[models]]
 id = "your-org/large-model"
-harness = "mistralrs"
+harness = "candle"
 quant = "Q4_K_M"
 vram_mb = 19000
 min_devices = 2
@@ -15,7 +15,7 @@ pinned_on = ["gpu-large"]
 
 [[models]]
 id = "your-org/medium-model"
-harness = "mistralrs"
+harness = "candle"
 quant = "Q6_K"
 vram_mb = 12000
 min_devices = 1
@@ -23,7 +23,7 @@ pinned_on = ["gpu-medium"]
 
 [[models]]
 id = "your-org/embedding-model"
-harness = "mistralrs"
+harness = "candle"
 quant = "Q8_0"
 vram_mb = 8000
 min_devices = 1