fix: 修正SubAgent调用所需的工具字段名称

- 将"Skills"字段更正为"Skill"，以确保与SubAgent调用的要求一致。

Author: BukeLy

agent-sdk-server/agent_session.py

@@ -158,7 +158,7 @@ async def process_message(
             #'Bash', 'Read', 'Write', 'Edit',
             #'Glob', 'Grep', 'WebFetch',
             'Task',
-            'Skills'  # Required for SubAgent invocation
+            'Skill'  # Required for SubAgent invocation
         ],
         mcp_servers=mcp_servers if mcp_servers else None,
         agents=agents if agents else None,