Verify self-contained schema preparation and submission

docs/consumers/pkg-promptkit.md

@@ -884,6 +884,12 @@ secret-handling policies.
 
 ## Extension Interfaces
 
+JSON Schema validation and native structured output use a self-contained root
+document. Internal references are supported; references to other files or
+network resources fail during preparation. `WithSchemaFile` selects one root
+file, while `WithSchemaFS` and `SchemaDir` select a root source. Prepared
+executions retain their captured schema even if the source later changes.
+
 Inject an [`LLMClient` or `ArtifactReader`](../../types.go) when the built-in
 behavior does not fit the application. Their GoDoc defines concurrent use,
 context handling, ownership of copied values, nil responses, and preservation