Preserve operation ownership in terminal observations

errors.go

@@ -13,6 +13,18 @@ import (
 	"go.maximumdirect.net/promptkit/internal/usecase"
 )
 
+// publicOperationError keeps classification separate from inspectable causes.
+// Cache wording so formatting nested operations does not repeatedly rebuild it.
+type publicOperationError struct {
+	owner   error
+	cause   error
+	message string
+}
+
+func (e *publicOperationError) Error() string        { return e.message }
+func (e *publicOperationError) Unwrap() error        { return e.cause }
+func (e *publicOperationError) Is(target error) bool { return errors.Is(e.owner, target) }
+
 func mapPublicError(err error) error {
 	if err == nil {
 		return nil
@@ -32,7 +44,7 @@ func mapPublicError(err error) error {
 			}
 			// The projection already retains ordinary causes; adding the original
 			// again would duplicate its entire subtree at each enclosing operation.
-			return errors.Join(owner, mapped)
+			return &publicOperationError{owner: owner, cause: mapped, message: fmt.Sprintf("%v: %v", owner, mapped)}
 		}
 	}
 	return mapUnownedPublicError(err)