Add immutable backend registry foundation

docs/policy/architecture.md

@@ -21,6 +21,8 @@ The implemented internal components consist of:
 
 - `internal/domain`, which owns framework data values shared by later internal
   components;
+- `internal/backend`, which owns validated immutable OpenAI-compatible backend
+  definitions and the built-in OpenRouter definition;
 - `internal/defaults`, which owns application-neutral framework defaults and
   constructs the default execution target;
 - `internal/filecatalog`, which discovers YAML files and provides source-path
@@ -72,9 +74,11 @@ downstream consumers, including Scriptorium
         narrow injected abstractions
 ```
 
-The facade coordinates internal components and adapts the supported public
-extension interfaces to narrow internal abstractions. Internal components must
-not depend on consumers or on Scriptorium.
+The backend registry depends on the domain model, and the model client reuses
+its OpenAI-compatible reserved request-field rule. The facade coordinates
+internal components and adapts the supported public extension interfaces to
+narrow internal abstractions. Internal components must not depend on consumers
+or on Scriptorium.
 
 ## Repository And Consumer Boundary