Harden backend contracts and documentation

docs/formats.md

@@ -168,6 +168,10 @@ variable name in `api_key_env`.
 
 Promptkit does not infer a backend from a model or endpoint. Endpoint-only
 profiles remain supported and have no effective backend ID.
+The engine always provides the built-in `openrouter` ID. Consumers can add
+engine-scoped IDs with
+[`WithBackend`](../backends.go); exact registration validation belongs to its
+GoDoc.
 
 `extra_params` accepts null, booleans, finite numbers, strings, arrays, and
 objects with string keys. Keys must be non-empty. With the built-in client,
@@ -226,8 +230,9 @@ invalid matching profile is an error and does not fall back. In-memory
 Every built-in selects the `openrouter` backend. The engine's built-in backend
 registry supplies `https://openrouter.ai/api/v1` and the environment-variable
 name `OPENROUTER_API_KEY`, so individual profiles contain only model and
-generation settings. A custom or in-memory profile with the same profile ID
-takes precedence.
+generation settings. Built-in profile files do not repeat those connection
+values. A custom or in-memory profile with the same profile ID takes
+precedence.
 
 | Provider | ID | Model |
 | --- | --- | --- |