fix(matrix): stop tagging the user on every reply (#16932)

The mention_user_id injection from #38a6bada9 unconditionally attached an
@user:server mention pill + MSC3952 m.mentions.user_ids payload to every
outbound reply and every tool-progress status update. The stated intent
was push notifications in muted rooms, but shipped as always-on in every
room, DM or group, muted or not — so every reply pinged the user.

- gateway/platforms/base.py: stop injecting mention_user_id into send
  metadata on every reply; restore the original _thread_metadata passthrough.
- gateway/run.py: drop mention_user_id from status-thread metadata.
- gateway/platforms/matrix.py: drop the mention-pill append block in
  _send_text that consumed the metadata. Keep the reaction-based exec
  approval half of #38a6bada9 and the inbound/outbound m.mentions
  handling (unrelated to the per-reply ping).

Reported by Elkim [NOUS] on Discord.

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

gateway/run.py

@@ -10033,7 +10033,7 @@ class GatewayRunner:
         # Bridge sync status_callback → async adapter.send for context pressure
         _status_adapter = self.adapters.get(source.platform)
         _status_chat_id = source.chat_id
-        _status_thread_metadata = {"thread_id": _progress_thread_id, "mention_user_id": source.user_id} if _progress_thread_id else {"mention_user_id": source.user_id}
+        _status_thread_metadata = {"thread_id": _progress_thread_id} if _progress_thread_id else None
 
         def _status_callback_sync(event_type: str, message: str) -> None:
             if not _status_adapter or not _run_still_current():