Is there a symbol I can use to have a where clause match all columns?
I do not agree with the approach you are thinking of using, but if you are dead set on using it, then I would probably structure the query like this:

SELECT path, markdown
  FROM public."Comment"
  where path = '/other/relative/path/' AND (TRUE OR markdown = 'here I am');


Essentially adding a 3rd parameter to your query .. one use case would use the 3rd param and set the 2nd param to FALSE, while another would set the 2nd param to TRUE..