docs: add a security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please report security issues **privately** using GitHub’s **Report a vulnerability** form on this repository (Security tab).
+
+**Do not** file public GitHub issues for security problems.
+
+When reporting, please include:
+- Affected project/repo and version(s)
+- Impact and component(s) involved
+- Reproduction steps or PoC (if available)
+- Your contact and preferred credit name
+
+If you do not receive an acknowledgement of your report within **6 business days**, or if you cannot find a private security contact for the project, you may **escalate to the OpenJS Foundation CNA** at `[email protected]`.
+
+If the project acknowledges your report but does not provide any further response or engagement within **14 days**, escalation is also appropriate.
+
+## Coordination & Disclosure
+
+We follow coordinated vulnerability disclosure:
+- We will acknowledge your report, assess impact, and work on a fix.
+- We aim to provide status updates at reasonable intervals until resolution.
+- We will publish a security advisory (and **CVE via the OpenJS CNA when applicable**) once a fix or mitigation is available. We credit reporters by default unless you request otherwise.