feat: respond to all messages in DMs without requiring @mention

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bot.py

@@ -279,7 +279,9 @@ class Bot:
             await self._send_text(room.room_id, HELP_TEXT)
             return
 
-        # Check if bot is mentioned (display name or user ID)
+        # In DMs (2 members), respond to all messages; in groups, require @mention
+        is_dm = room.member_count == 2
+        if not is_dm:
             bot_display = self.client.user_id.split(":")[0].lstrip("@")
             mentioned = (
                 BOT_USER in body
@@ -405,11 +407,15 @@ class Bot:
         )
 
     async def on_megolm(self, room, event: MegolmEvent):
-        """Log undecryptable messages."""
+        """Request keys for undecryptable messages."""
         logger.warning(
-            "Undecryptable event %s in %s from %s",
+            "Undecryptable event %s in %s from %s — requesting keys",
             event.event_id, room.room_id, event.sender,
         )
+        try:
+            await self.client.request_room_key(event)
+        except Exception:
+            logger.debug("Key request failed", exc_info=True)
 
     async def on_key_verification(self, event):
         """Auto-accept key verification requests."""