Clarify artifact reader ownership transfer

docs/consumers/pkg-promptkit.md

@@ -891,6 +891,11 @@ of collaborator errors. Implementations must honor cancellation, safely manage
 copies they retain, avoid unsafe logging of content or credentials, and enforce
 the application policy that motivated the injection.
 
+A successful `ArtifactReader.Read` transfers the returned artifact and mutable
+body storage to Promptkit. Readers that previously reused a returned buffer
+must instead return independent storage and stop mutating it after return;
+private scratch buffers may still be reused when the returned bytes are copied.
+
 ## Handle Errors
 
 Use `errors.Is` with the

docs/roadmap/implementation.md

@@ -244,6 +244,10 @@ Focused authentication and public smoke tests pass on Go 1.27.3.
 the accepted handoff, without promising safety for contract-violating readers.
 Explain migration in consumer guidance; no borrowing/release protocol is added.
 
+Completion: ArtifactReader ownership and migration guidance require independent
+transferred storage. Focused concurrent preparation, adapter isolation, and
+cause-propagation tests pass with the race detector on Go 1.27.3.
+
 ## Stage 9: Detach Prepared Ownership And Session Storage
 
 **Catalog:** 16, 18. **Files:** runner/prepared state and domain session normalization.