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

README.md

@@ -37,7 +37,7 @@ Plano pulls rote plumbing out of your framework so you can stay focused on what
 **Jump to our [docs](https://docs.planoai.dev)** to learn how you can use Plano to improve the speed, safety and obervability of your agentic applications.
 
 > [!IMPORTANT]
-> Plano and the Arch family of LLMs (like Plano-Orchestrator-4B, Arch-Router, etc) are hosted free of charge in the US-central region to give you a great first-run developer experience of Plano. To scale and run in production, you can either run these LLMs locally or contact us on [Discord](https://discord.gg/pGZf2gcwEc) for API keys.
+> Plano and the Plano family of LLMs (like Plano-Orchestrator) are hosted free of charge in the US-central region to give you a great first-run developer experience of Plano. To scale and run in production, you can either run these LLMs locally or contact us on [Discord](https://discord.gg/pGZf2gcwEc) for API keys.
 
 ---