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/agents/requirements.txt

@@ -0,0 +1,19 @@
+# X-Agents Agent Core Dependencies
+
+# Web framework
+fastapi>=0.109.0
+uvicorn>=0.27.0
+pydantic>=2.5.0
+
+# LLM providers
+openai>=1.12.0
+anthropic>=0.18.0
+
+# Async
+aiohttp>=3.9.0
+
+# Vector search (optional)
+chromadb>=0.4.0
+
+# Utilities
+python-dotenv>=1.0.0