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/resources/includes/plano_config_agents_filters.yaml

@@ -30,7 +30,7 @@ listeners:
   - type: agent
     name: agent_1
     port: 8001
-    router: arch_agent_router
+    router: plano_agent_router
     agents:
       - id: rag_agent
         description: virtual assistant for retrieval augmented generation tasks

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -1,4 +1,4 @@
-# Arch Gateway configuration version
+# Plano Gateway configuration version
 version: v0.3.0
 
 # External HTTP agents - API type is controlled by request path (/v1/responses, /v1/messages, /v1/chat/completions)