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

cli/planoai/config_generator.py

@@ -372,16 +372,15 @@ def validate_and_render_schema():
     # Build lookup of model names (already prefix-stripped by config processing)
     model_name_set = {mp.get("model") for mp in updated_model_providers}
 
-    # Auto-add arch-router provider if routing preferences exist and no provider matches the router model
-    router_model = overrides_config.get("llm_routing_model", "Arch-Router")
-    # Strip provider prefix for comparison since config processing strips prefixes from model names
+    # Auto-add plano-orchestrator provider if routing preferences exist and no provider matches the routing model
+    router_model = overrides_config.get("llm_routing_model", "Plano-Orchestrator")
     router_model_id = (
         router_model.split("/", 1)[1] if "/" in router_model else router_model
     )
     if len(model_usage_name_keys) > 0 and router_model_id not in model_name_set:
         updated_model_providers.append(
             {
-                "name": "arch-router",
+                "name": "plano-orchestrator",
                 "provider_interface": "plano",
                 "model": router_model_id,
                 "internal": True,