Bound external result file reads

internal/stage/trim.go

@@ -3,7 +3,6 @@ package stage
 import (
 	"context"
 	"fmt"
-	"os"
 	"path/filepath"
 	"strings"
 	"time"
@@ -389,9 +388,9 @@ func resolveTrimmedOutputPath(paths artifacts.SessionPaths, cfg *config.TrimConf
 }
 
 func copyTranscript(store artifacts.Store, src, dst string) error {
-	data, err := os.ReadFile(src)
+	data, err := readExternalResult(src, "seriatim trimmed transcript result")
 	if err != nil {
-		return fmt.Errorf("read source transcript: %w", err)
+		return err
 	}
 	if err := store.WriteFileAtomic(dst, data, fileops.WorkspaceFileMode); err != nil {
 		return fmt.Errorf("write destination transcript: %w", err)