`labels <number>` only lists an issue/PR's labels; there was no way to
list the project-wide label set, which triage/labelling needs to discover
valid names. Add `repo-labels [pattern]` hitting GET /repos/{owner}/{repo}/labels
(paginated), with optional name-substring filter. Default prints one label
per line (name + tab-separated description when set); --json emits the full
label objects.
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