cmux: show images in a split without stealing focus

In cmux, cmux open <file> adds a preview tab to the caller's own pane, so it doesn't create a side-by-side view. To get a split, run cmux new-pane --type terminal --direction right --focus false, then cmux open <png> --pane <pane> --no-focus, then cmux close-surface on the new pane's empty terminal tab. cmux tree shows the pane and surface refs. This keeps the user in the terminal, where macOS open would send them to another app.