Document structured generation errors

docs/consumers/pkg-promptkit.md

@@ -450,6 +450,24 @@ status; those choices remain with the consuming application. The
 contract, while the [`Engine.Run` and error GoDoc](../../engine.go) owns broad
 error and cancellation identities.
 
+For a non-2xx response from the built-in OpenAI-compatible client, inspect the
+status and deliberately selected provider diagnostic when useful:
+
+```go
+var generationErr *promptkit.GenerationError
+if errors.As(err, &generationErr) {
+	status := generationErr.StatusCode()
+	message := generationErr.ProviderMessage()
+	_, _ = status, message // Apply application retry and presentation policy.
+}
+```
+
+All provider fields are untrusted and can contain sensitive request or schema
+fragments. Do not log, display, or return them without an application-specific
+disclosure policy. Promptkit does not assign retry or presentation behavior.
+The [`GenerationError` GoDoc](../../generation_error.go) owns the exact typed
+error contract.
+
 ## Application Boundary
 
 Promptkit is an importable library. It does not own a command, inbound HTTP