fix(09-04): keep relation columns out of the default sort set

- A relation column is not sortable unless columns.yaml says so
- An explicit sortable relation orders the joined select column, then the primary key

cabana/list_schema.go

@@ -227,7 +227,7 @@ func compileColumns(pluginID string, ctl pact.AdminController, fsys fs.FS, colPa
 				return nil, bootErr(pluginID, ctl.ID(), colPath, fmt.Errorf("column %s is not on the model", key))
 			}
 		}
-		sortable := true
+		sortable := spec.Relation == ""
 		if spec.Sortable != nil {
 			sortable = *spec.Sortable
 		}