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/guides/includes/config.yaml

@@ -24,7 +24,7 @@ prompt_targets:
       name: app_server
       path: /agent/summary
       http_method: POST
-    # 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.
@@ -46,7 +46,7 @@ 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

docs/source/guides/observability/access_logging.rst

@@ -1,4 +1,4 @@
-.. _arch_access_logging:
+.. _plano_access_logging:
 
 Access Logging
 ==============

docs/source/guides/observability/tracing.rst

@@ -1,4 +1,4 @@
-.. _arch_overview_tracing:
+.. _plano_overview_tracing:
 
 Tracing
 =======