fix: explicit m.mentions for ticket notifications and roasts (ahead of Synapse 1.164)

Synapse 1.164 turns on MSC4210 by default: the legacy push rules that
pinged when a user ID or display name appeared in the body are removed, so
a message only notifies the users listed in m.mentions.

- hookshot/tinker-tickets.js deliberately pings assignees/watchers
  (notify_users) with matrix.to links; those pings would stop. Return
  mentions: { user_ids: notifyUsers } (hookshot 7.3.2 v2 transforms
  support it).
- matrixbot: send_html takes optional mention_user_ids; !roast mentions the
  target when given as a full Matrix ID. Winner announcements stay without
  a ping (the player is already in the room).

No @room is sent anywhere. Lotus Chat already sends m.mentions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

hookshot/tinker-tickets.js

@@ -77,4 +77,13 @@ if (event === 'ticket_created') {
   html  = '[tinker-tickets] event: <code>' + event + '</code> ticket #' + id;
 }
 
-result = { version: 'v2', plain: plain, html: html, msgtype: 'm.text' };
+// Explicit mentions (m.mentions): from Synapse 1.164 the legacy push rules
+// that pinged on a user ID or display name in the body are off by default
+// (MSC4210), so the people to notify must be listed here.
+result = {
+  version: 'v2',
+  plain: plain,
+  html: html,
+  msgtype: 'm.text',
+  mentions: { user_ids: notifyUsers }
+};