Publish effective LLM backend provenance

docs/integrations/pkg-promptkit.md

@@ -17,7 +17,7 @@ Notarius relies on the root `promptkit` package to:
 - prepare and run a `RunRequest` with named inline artifacts, variables,
   a direct session ID, prompt identity, and profile selection;
 - return rendered debug material, validated structured output, selected
-  profile and model metadata, and token usage;
+  profile, backend, effective model metadata, and token usage;
 - distinguish structured-output validation failure from execution failure; and
 - identify a missing explicit profile through `ErrProfileNotFound`.
 
@@ -33,6 +33,13 @@ the same value as the `session_id` prompt variable for maintained prompt
 compatibility. Session IDs are stable, non-secret correlation identifiers and
 may be exposed to providers and provider observability.
 
+Notarius records PromptKit's selected backend ID and effective reasoning
+setting as optional run-manifest provenance. Endpoint-only profiles have no
+backend ID. Debug prompt material also retains the selected backend ID and
+PromptKit's stable lower-case `effective_model_params` JSON, which may include
+`backend_id`. Notarius production configuration does not expose user-defined
+PromptKit backend registration.
+
 ## Notarius Ownership
 
 [LLM Runtime Internals](../internal/llm.md) describes how Notarius mounts