Publish the Promptkit engine facade

docs/integrations/openai-compatible-chat.md

@@ -5,8 +5,8 @@
 This document defines the outbound HTTP behavior implemented by Promptkit's
 internal OpenAI-compatible model client. The
 [internal model-client document](../internal/llm.md) owns implementation flow,
-errors, and test ownership. The client is not yet available through a usable
-public Promptkit engine.
+errors, and test ownership. The root Promptkit engine uses this client by
+default unless a consumer injects another implementation.
 
 ## Endpoint And Method