repo mirror list: replace generic sortRows with concrete sorters

Sorting only ever covers two fixed 3-column sets, so the generic sortRows[T]/sortMirrorsDefault split wasn't earning its complexity and had two ordering bugs:

• desc was read from the raw spec but the column name from the trimmed spec, and the two call sites passed raw vs trimmed, so --sort " -repo" behaved differently between the mirror and --show-available lists.
• only default/--sort repo got the (owner/repo, clusterHost) tiebreak; every other column fell back to a single-key sort, leaving duplicate-key rows in arbitrary server order across runs.

Replace with a shared parseSortColumn (trims before reading direction) plus concrete sortMirrors/sortAvailable that always apply the owner/repo(+cluster) tiebreak, so every column is deterministic and whitespace is handled the same on both paths.