Route container image save reference list to stderr in stdout mode (#1804)

- Fixes #1801.
- When `container image save` runs without `--output`,
  stdout carries the OCI tar archive. The command writes 
  the archive bytes to stdout and then `print(reference)`s
  each saved image reference to stdout afterward,
  appending non-archive text after the tar EOF marker,
  which will cause strict tar/OCI consumers to fail.
- This routes the saved-reference list to stderr in the
  no-`--output` branch, so stdout contains only archive
  bytes. When saving to a file via `--output`, stdout is
  free, so the references continue to print to stdout
  exactly as before.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Sources/ContainerCommands/Image/ImageSave.swift

@@ -140,7 +140,15 @@ extension Application {
 
             progress.finish()
             for reference in references {
-                print(reference)
+                if output == nil {
+                    // stdout is carrying the OCI archive in this branch, so the
+                    // saved-reference list goes to stderr via the logger. Printing
+                    // it to stdout appends non-archive bytes after the tar EOF and
+                    // corrupts the stream for redirection and pipelines (#1801).
+                    log.info("\(reference)")
+                } else {
+                    print(reference)
+                }
             }
         }
     }