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/platforms/base.py

@@ -2469,15 +2469,11 @@ class BasePlatformAdapter(ABC):
                 # Send the text portion
                 if text_content:
                     logger.info("[%s] Sending response (%d chars) to %s", self.name, len(text_content), event.source.chat_id)
-                    # Build send metadata: thread_id + mention target for platforms that need it
-                    send_metadata = dict(_thread_metadata) if _thread_metadata else {}
-                    if event.source.user_id:
-                        send_metadata["mention_user_id"] = event.source.user_id
                     result = await self._send_with_retry(
                         chat_id=event.source.chat_id,
                         content=text_content,
                         reply_to=event.message_id,
-                        metadata=send_metadata,
+                        metadata=_thread_metadata,
                     )
                     _record_delivery(result)