Execute PromptKit requests from prepared snapshots

docs/integrations/pkg-promptkit.md

@@ -14,8 +14,9 @@ Notarius relies on the root `promptkit` package to:
 
 - construct an `Engine` with filesystem-backed prompt, schema, and optional
   profile sources;
-- prepare and run a `RunRequest` with named inline artifacts, variables,
-  a direct session ID, prompt identity, and profile selection;
+- prepare one frozen execution from a `RunRequest` with named inline artifacts,
+  variables, a direct session ID, prompt identity, and profile selection, then
+  record credential-redacted details and run that exact execution;
 - return rendered debug material, validated structured output, selected
   profile, backend, effective model metadata, and token usage;
 - register the optional conventional `local` backend through `BackendLocal`,
@@ -58,8 +59,9 @@ Notarius retains its application-wide scheduled client around the PromptKit
 adapter. PromptKit may apply a narrower limit for the selected backend;
 endpoint-only profiles have no such backend limit. The adapter translates
 PromptKit capacity rejection into the provider-neutral Notarius
-`ErrLLMCapacityExceeded` contract and leaves retries to the calling pipeline
-stage.
+`ErrLLMCapacityExceeded` contract. It may include the normalized selected
+backend ID in safe diagnostic context, without exposing PromptKit's capacity
+error type, and leaves retries to the calling pipeline stage.
 
 ## Notarius Ownership