repo mirror list: split column key from display header

Replace the parseSortColumn parenthetical-stripping (regexp + sortKeyOf) with a column{key, header} struct: key is the canonical --sort/match name, header is the table text. They differ only where the header adds a display hint the key shouldn't (NAME -> "NAME (owner/repo)"), so --sort matching is a plain equality on key with no header parsing.

--sort now takes the key ("name"); the full display form is no longer a second accepted spelling (it never needed to be — it was only a workaround for overloading one string for both jobs).