Make mautrix-gmessages honor matrix_bridge_mautrix_gmessages_command_prefix

The variable was defined, but the configuration template hardcoded the
command prefix, so changing the variable had no effect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

roles/custom/matrix-bridge-mautrix-gmessages/templates/config.yaml.j2

@@ -24,7 +24,7 @@ network:
 # Config options that affect the central bridge module.
 bridge:
     # The prefix for commands. Only required in non-management rooms.
-    command_prefix: '!gm'
+    command_prefix: {{ matrix_bridge_mautrix_gmessages_command_prefix | to_json }}
     # Should the bridge create a space for each login containing the rooms that account is in?
     personal_filtering_spaces: {{ matrix_bridge_mautrix_gmessages_bridge_personal_filtering_spaces | to_json }}
     # Whether the bridge should set names and avatars explicitly for DM portals.