hive-forge: fetch issue comments in one request; the endpoint has no paging

The forge's issue-comments endpoint takes no page or limit and always
returns the whole thread. #5055 wrapped view's fetch in all_pages, which
never saw a short page on a thread of 50+ comments and fetched the same
list 50 times: `issue view 3782` printed 3000 comments instead of 60.
`comments --limit K` sent K as a page size the forge ignored, so it
returned the whole thread.

view and pr status's last_comment now send one request. comments fetches
the thread once and cuts the --limit / --tail window client-side; the
X-Total-Count probe and the computed-page tail walk are gone, and the
count of comments outside the window is the length of the list.
--since is unchanged.

tests/comments_no_paging.rs runs the binary against a loopback mock
forge whose comments route ignores page and limit, as Forgejo's does.

docs/tools/forge.md

@@ -42,7 +42,7 @@ hive-forge issue create --title "..." --body "..."
 # --- flat aliases below remain valid (hidden) ---
 hive-forge view 42                          # title + body + comments (marks thread read)
 hive-forge comments 42                      # list all comments (human-readable; marks thread read); for PRs, review bodies merged in
-hive-forge comments 42 --tail 10            # last 10 comments (count-then-page; efficient on long threads)
+hive-forge comments 42 --tail 10            # last 10 comments (fetches the whole thread, shows the tail)
 hive-forge --json comments 42               # same as above, JSON array (global flag)
 hive-forge comment 42 --body "..."          # post comment (inline body)
 hive-forge comment 42 --body-file - <<EOF   # ...or pipe a HEREDOC