use plano-orchestrator for LLM routing, remove arch-router

Replace RouterService/RouterModelV1 (arch-router prompt) with
OrchestratorService/OrchestratorModelV1 (plano-orchestrator prompt)
for LLM routing. This ensures the correct system prompt is used when
llm_routing_model points at a Plano-Orchestrator model.

- Extend OrchestratorService with session caching, ModelMetricsService,
  top-level routing preferences, and determine_route() for LLM routing
- Delete RouterService, RouterModel trait, RouterModelV1, and
  ARCH_ROUTER_V1_SYSTEM_PROMPT
- Unify defaults to Plano-Orchestrator / plano-orchestrator
- Update CLI config generator, demos, docs, and config schema

Made-with: Cursor

demos/llm_routing/codex_router/pretty_model_resolution.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # Pretty-print Plano MODEL_RESOLUTION lines from docker logs
-# - hides Arch-Router
+# - hides Plano-Orchestrator
 # - prints timestamp
 # - colors MODEL_RESOLUTION red
 # - colors req_model cyan
@@ -9,7 +9,7 @@
 
 docker logs -f plano 2>&1 \
 | awk '
-/MODEL_RESOLUTION:/ && $0 !~ /Arch-Router/ {
+/MODEL_RESOLUTION:/ && $0 !~ /Plano-Orchestrator/ {
   # extract timestamp between first [ and ]
   ts=""
   if (match($0, /\[[0-9-]+ [0-9:.]+\]/)) {