完善文档

docs/architecture/system.md

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+# 系统架构
+
+## 核心模块
+
+- Client
+- BFF
+- Agent Runtime
+- Model Gateway
+- Memory Manager
+- Planning Engine
+- Tool Runtime
+- RAG
+- Event System
+- Data Layer
+- Observability
+
+## 调用关系
+
+```mermaid
+flowchart LR
+    Client --> BFF
+    BFF --> Runtime
+    Runtime --> Model
+    Runtime --> Memory
+    Runtime --> Planner
+    Planner --> Tools
+    Tools --> External
+    Runtime --> Events
+    Runtime --> Observability
+```
+
+模块之间优先使用明确接口，不允许跨层直接访问内部实现。