Catch the conventions up to the cluster the filter takes

This branch moved the CLUSTER column onto the public host and made repo mirror list --cluster match it, but the conventions still described the old arrangement twice: that the column prints the slug that filter takes, and that the filter is resolved server-side and accepts either spelling. It is neither. It is a client-side filter over the rows the command prints, which is precisely why the column had to move — a spelling the column does not print matches nothing at all, silently.