feat(mcp): load MCP servers asynchronously at startup

Boot MCP servers (npx/uvx) on a background thread instead of blocking
agent init. Built-in tools serve traffic immediately while MCP comes
online; each new agent reads whatever is ready at creation time.
Idempotent via _mcp_loaded flag — concurrent sessions never re-fork
subprocesses. Per-server failures are isolated and warmup is triggered
in app.py so loading overlaps with channel startup.

config.py

@@ -246,15 +246,9 @@ class Config(dict):
         self.user_datas = {}
 
     def __getitem__(self, key):
-        # 跳过以下划线开头的注释字段
-        if not key.startswith("_") and key not in available_setting:
-            logger.debug("[Config] key '{}' not in available_setting, may not take effect".format(key))
         return super().__getitem__(key)
 
     def __setitem__(self, key, value):
-        # 跳过以下划线开头的注释字段
-        if not key.startswith("_") and key not in available_setting:
-            logger.debug("[Config] key '{}' not in available_setting, may not take effect".format(key))
         return super().__setitem__(key, value)
 
     def get(self, key, default=None):
@@ -262,7 +256,7 @@ class Config(dict):
         if key.startswith("_"):
             return super().get(key, default)
         
-        # 如果key不在available_setting中，直接返回default
+        # 如果key不在available_setting中，直接走dict的get，返回config.json中实际加载的值（如不存在则返回default）
         if key not in available_setting:
             return super().get(key, default)