docs(security): add SECURITY.md

Vulnerability reporting policy for the C-axis trust program: private
disclosure address, response process, and an honest pre-1.0 supported-
versions statement (no fabricated version table, no SBOM claim).

SECURITY.md

@@ -0,0 +1,33 @@
+# 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-mailbox
+
+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
+
+This project is pre-1.0 (a single continuous 0.x line, currently in the
+0.25 series) and carries no parallel maintenance branches. Only the
+latest tagged release receives security fixes; please upgrade to the
+latest release before reporting.
+
+## Advisories
+
+Security-relevant fixes are recorded in [CHANGELOG.md](CHANGELOG.md).
+Given the project's current scale, we do not yet publish separate
+signed advisories.