repo mirror list: use shell-friendly kebab sort keys
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Sort keys become lower-case single tokens (clone-url, not "CLONE URL"),
so --sort clone-url needs no quoting. Headers keep their upper-case
display text. The unknown-column error now lists the friendly keys.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: 01KX0N45RGEWPVEQR2CHC2M90P
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