Translate LLM backend capacity failures

docs/integrations/pkg-promptkit.md

@@ -19,7 +19,8 @@ Notarius relies on the root `promptkit` package to:
 - return rendered debug material, validated structured output, selected
   profile, backend, effective model metadata, and token usage;
 - distinguish structured-output validation failure from execution failure; and
-- identify a missing explicit profile through `ErrProfileNotFound`.
+- identify a missing explicit profile through `ErrProfileNotFound` and backend
+  admission exhaustion through `ErrCapacityExceeded`.
 
 Notarius does not use PromptKit's optional `ArtifactReader`. It materializes
 source and reference content itself and supplies owned inline artifacts at the
@@ -40,6 +41,13 @@ 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 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.
+
 ## Notarius Ownership
 
 [LLM Runtime Internals](../internal/llm.md) describes how Notarius mounts