Mirror the #tag mark so the TipTap editor styles @username while typing
and on load, matching the read-only view. A shared mention grammar
(utils/mention-grammar.ts) feeds both the editor tokenizer and the
read-only remark renderer so they can't drift.
Styling only -- no autocomplete dropdown (ListUsers is admin-only and
there is no user-search RPC). Bare emails still autolink to mailto: and
are never treated as mentions: the tokenizer's start() skips email-glued
@s so marked's GFM autolinker keeps seeing the whole address.
