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/model_routing_service/config_k8s.yaml

@@ -1,7 +1,7 @@
 version: v0.3.0
 
 overrides:
-  llm_routing_model: plano/Arch-Router
+  llm_routing_model: plano/Plano-Orchestrator
 
 listeners:
   - type: model
@@ -10,8 +10,8 @@ listeners:
 
 model_providers:
 
-  - model: plano/Arch-Router
-    base_url: http://arch-router:10000
+  - model: plano/Plano-Orchestrator
+    base_url: http://plano-orchestrator:10000
 
   - model: openai/gpt-4o-mini
     access_key: $OPENAI_API_KEY