Update remaining arch references in docs

- Rename RST cross-reference labels: arch_access_logging, arch_overview_tracing, arch_overview_threading → plano_*
- Update label references in request_lifecycle.rst
- Rename arch_config_state_storage_example.yaml → plano_config_state_storage_example.yaml
- Update config YAML comments: "Arch creates/uses" → "Plano creates/uses"
- Update "the Arch gateway" → "the Plano gateway" in configuration_reference.rst
- Update arch_config_schema.yaml reference in provider_models.py
- Rename arch_agent_router → plano_agent_router in config example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/source/concepts/includes/plano_config.yaml

@@ -18,7 +18,7 @@ prompt_targets:
     endpoint:
       name: app_server
       path: /agent/summary
-    # Arch uses the default LLM and treats the response from the endpoint as the prompt to send to the LLM
+    # Plano uses the default LLM and treats the response from the endpoint as the prompt to send to the LLM
     auto_llm_dispatch_on_response: true
     # override system prompt for this prompt target
     system_prompt: You are a helpful information extraction assistant. Use the information that is provided to you.
@@ -39,7 +39,7 @@ prompt_targets:
         default: false
         enum: [true, false]
 
-# Arch creates a round-robin load balancing between different endpoints, managed via the cluster subsystem.
+# Plano creates a round-robin load balancing between different endpoints, managed via the cluster subsystem.
 endpoints:
   app_server:
     # value could be ip address or a hostname with port