The forge clamps page_size to 50. branches, pr reviews, the review
bodies merged into `comments <pr>`, and view's comment list each sent
one request, so anything past the first page was dropped silently.
labels::repo_labels did loop, but asked for 100/page and stopped on
`len < 100`, so a clamped full page of 50 ended the walk.
Add verbs::all_pages: requests 50/page until a short page, capped at
50 pages with a stderr warning when the cap is hit. Use it in all of
the above; repo-labels now calls labels::repo_labels instead of its
own copy of the loop.
list --json now prints the same "N of M shown" stderr trailer as the
pretty output; its paging and stdout shape are unchanged.
Closes#5051
`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.
