feat: support disabled_adapters list in system config

Adds 'system.disabled_adapters' config option (list of adapter names).
Disabled adapters are excluded from both the adapter registry and API
responses, preventing users from creating bots with those adapters.

Example config:
  system:
    disabled_adapters:
      - aiocqhttp
      - dingtalk

src/langbot/templates/config.yaml

@@ -20,6 +20,7 @@ system:
     edition: community
     recovery_key: ''
     allow_modify_login_info: true
+    disabled_adapters: []
     limitation:
         max_bots: -1
         max_pipelines: -1