Add security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please **do not** open a public GitHub issue for security vulnerabilities.
+
+Report security issues privately using GitHub's built-in advisory mechanism:
+
+**[Security → Advisories → New draft advisory](../../security/advisories/new)**
+
+Include:
+- A description of the vulnerability
+- The file and line number where it occurs
+- Steps to reproduce or a proof-of-concept
+- Your assessment of impact and severity
+
+We aim to acknowledge reports within **3 business days** and provide a fix or mitigation plan within **30 days** for confirmed vulnerabilities.
+
+Responsible disclosure is appreciated. We will credit researchers in the release notes unless you prefer to remain anonymous.
+
+## Scope
+
+This plugin requires WordPress Application Password authentication for all tool calls. There are no unauthenticated attack surfaces. Vulnerabilities affecting only users with `manage_options` (Administrator role) are considered lower severity than those affecting `edit_posts` (Editor/Author role).
+
+## Supported Versions
+
+Security fixes are applied to the latest release only.