Setting buffer.tempfile = 'prompt.md' pushed prompt_toolkit into its
complex-tempfile path, which creates a temp dir and then calls
os.makedirs() on that same path when no subdirectory is present. That
raises EEXIST before the editor can launch.
Keep prompt_toolkit on the simple tempfile path with .md suffix, and
make the editor fallback chain explicit on both surfaces:
$VISUAL -> $EDITOR -> nvim -> vim -> vi -> nano.