完善文档

docs/security/overview.md

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+# Security
+
+采用纵深防御：
+
+`Identity -> Authentication -> Authorization -> Tool Permission -> Sandbox -> Audit`
+
+重点防护：
+
+- Prompt Injection
+- Tool Abuse
+- Secret Leakage
+- Sensitive Data
+- Sandbox Escape
+- Supply Chain Risk