Translate LLM backend capacity failures

docs/internal/llm.md

@@ -81,6 +81,13 @@ worker counts cannot exceed the configured LLM limit. The configuration field
 and its effective default are owned by
 [Configuration](../config.md#concurrency-output-cache-and-debug).
 
+PromptKit applies a second, independent admission limit when the selected
+profile names a limited backend. It sits beneath the Notarius scheduled client,
+so it may narrow but cannot expand the application-wide limit. Built-in
+OpenRouter profiles select PromptKit's reserved backend and its upstream
+capacity policy. Endpoint-only profiles do not select a PromptKit backend and
+remain limited only by the Notarius scheduler.
+
 ## Prompt And Schema Assets
 
 An `AssetRegistry` collects prompt and schema filesystems from production module
@@ -132,6 +139,14 @@ failure, empty structured body, or decode failure as
 material when they exist. Provider failures remain operational errors rather
 than output-validation failures.
 
+When PromptKit rejects backend admission before generation, the adapter maps
+`promptkit.ErrCapacityExceeded` to
+`contracts.ErrLLMCapacityExceeded`, retaining prompt context and a redacted
+upstream diagnostic without exposing the PromptKit sentinel as a framework
+contract. A canceled caller context takes precedence. The adapter does not
+retry capacity failures; the pipeline's existing binding attempt policy sees
+the operational error and decides whether to rerun the complete operation.
+
 Prompt-declared repair is executed within PromptKit’s structured-output flow.
 The current production D&D prompt manifests set repair attempts to zero. That
 setting does not replace pipeline retry behavior: a binding’s configured retry
@@ -184,6 +199,8 @@ contents safe for general logging.
   sources, invalid asset registration, or a non-positive scheduler limit.
 - Preparation failures, unavailable explicit profiles, provider failures, and
   context cancellation propagate to the calling stage with context.
+- Backend admission exhaustion is a provider-neutral operational error and is
+  not classified as invalid structured output or validator rejection.
 - Malformed or schema-invalid provider output is classified separately as
   invalid structured output so the module or pipeline can apply its own retry
   and rejection policy.