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/preference_based_routing/README.md

@@ -32,9 +32,9 @@ planoai up config.yaml
 
 3. Test with curl or open AnythingLLM http://localhost:3001/
 
-## Running with local Arch-Router (via Ollama)
+## Running with local routing model (via Ollama)
 
-By default, Plano uses a hosted Arch-Router endpoint. To self-host Arch-Router locally using Ollama:
+By default, Plano uses a hosted Plano-Orchestrator endpoint. To self-host a routing model locally using Ollama:
 
 1. Install [Ollama](https://ollama.ai) and pull the model:
 ```bash