hive-forge: page past the forge's 50-row clamp; show list --json truncation

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

docs/tools/forge.md

@@ -336,8 +336,8 @@ to discover valid label names before triaging or to audit the label set.
   isn't a current member isn't necessarily a typo.
 - `list --limit N` is a *request*: the forge clamps page size to its own
   `api.MAX_RESPONSE_ITEMS` (50 by default), so `--limit 400` returns at
-  most 50 rows. The stderr trailer reports the real total from the
-  response's `X-Total-Count` (`… 50 of 187 shown … 137 more`), so trust
-  the trailer, not the row count, when deciding whether you've seen
-  everything.
+  most 50 rows. The stderr trailer (printed in `--json` mode too)
+  reports the real total from the response's `X-Total-Count`
+  (`… 50 of 187 shown … 137 more`), so trust the trailer, not the row
+  count, when deciding whether you've seen everything.
 <!-- vale write-good.Passive = YES -->