feat: 新增 core/agents 模块和 nanobot

- 新增 agents 模块，包含 agent、api、skills 等子模块
- 新增 nanobot 项目，支持多渠道集成
- 添加启动脚本 start-all.bat 和 start-all.sh

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

core/nanobot/tests/test_message_tool.py

@@ -0,0 +1,10 @@
+import pytest
+
+from nanobot.agent.tools.message import MessageTool
+
+
+@pytest.mark.asyncio
+async def test_message_tool_returns_error_when_no_target_context() -> None:
+    tool = MessageTool()
+    result = await tool.execute(content="test")
+    assert result == "Error: No target channel/chat specified"