完善文档

docs/agent/tool-system.md

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+# Tool System
+
+## 架构
+
+```mermaid
+flowchart LR
+    Registry["Tool Registry"] --> Discovery["Discovery"]
+    Discovery --> Permission["Permission"]
+    Permission --> Validation["Schema Validation"]
+    Validation --> Runtime["Execution"]
+    Runtime --> Normalize["Result Normalize"]
+    Normalize --> Audit["Audit"]
+```
+
+## Tool 必须具备
+
+- name
+- description
+- input schema
+- output schema
+- permission
+- timeout
+- retry policy
+- version
+
+## 统一原则
+
+Agent 不直接执行任意外部能力。所有工具调用必须经过 Tool Runtime。