docs: add SECURITY.md

AAA+ program round 3 (C-axis, trust). No file existed before; new
vulnerability-reporting policy at repo root, matching the org-wide
10/10 template from the coordinated round.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VF8cJYs2vZazMisAd95nfM

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/repo-standard
+
+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 |
+| ------- | --------- |
+| 0.11.x  | :white_check_mark: |
+| < 0.11  | :x:       |
+
+This project has not reached 1.0 yet; only the latest 0.x release line
+receives security fixes. See `CHANGELOG.md` for release history.
+
+## Advisories
+
+No advisories have been published yet.