docs: add security policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported |
+|---------|-----------|
+| latest  | Yes       |
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability, please report it responsibly via email:
+
+**jack@yuanasi.com**
+
+Please do **not** open a public GitHub issue for security vulnerabilities.
+
+We will acknowledge receipt within 48 hours and aim to provide a fix or mitigation plan within 7 days.