docs: add SECURITY.md with vulnerability reporting policy

Reporting address, canonical repo, response process, and a supported-
versions table anchored to the current v5.13.0 tag.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K9Kp4MhrqsJRbcGyjS3TnP

SECURITY.md

@@ -0,0 +1,34 @@
+# Security policy
+
+## Reporting a vulnerability
+
+Report privately to <security@fromaitochitta.com> - do not open a
+public issue.
+Canonical repository: https://git.fromaitochitta.com/open/config-audit
+
+Please include the affected version or commit, a minimal reproduction,
+and the impact you see. We acknowledge every report within 5 working
+days, agree a fix and disclosure timeline with the reporter, and aim to
+disclose within 90 days of the initial report.
+
+## Response process
+
+1. Acknowledge within 5 working days.
+2. Triage and confirm severity within 10 working days.
+3. Develop and test a fix.
+4. Publish an advisory and credit the reporter unless they prefer
+   to remain anonymous.
+
+## Supported versions
+
+| Version | Supported |
+|---------|-----------|
+| 5.13.x  | Yes |
+| < 5.13  | No  |
+
+Only the latest tagged release receives security fixes. There is no
+long-term support line.
+
+## Advisories
+
+Security-relevant fixes are noted in [CHANGELOG.md](CHANGELOG.md).