chore: add canonical SECURITY.md per outbound disclosure policy

Adopts the canonical SECURITY.md template published in
Stackbilt-dev/docs#15 and served at
https://docs.stackbilt.dev/ecosystem/SECURITY.md.template.

Points disclosure traffic at admin@stackbilt.dev and this repo's
GitHub Security Advisory endpoint, matching the policy's channel
routing matrix for public Stackbilt-dev repositories.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Codebeast

SECURITY.md

@@ -0,0 +1,45 @@
+# Security
+
+For the full Stackbilt security policy, see https://docs.stackbilt.dev/security/.
+
+## Reporting a Vulnerability
+
+**Do not open a public GitHub issue for security vulnerabilities.**
+
+### How to report
+
+- **Primary channel:** email `admin@stackbilt.dev` with "SECURITY:" in the subject line
+- **GitHub Security Advisory:** https://github.com/Stackbilt-dev/stackbilt-mcp-gateway/security/advisories/new
+- Include: vulnerability description, reproduction steps, potential impact, and any suggested mitigation
+
+### Response targets
+
+| Severity | Acknowledgement | Fix target |
+|---|---|---|
+| Critical — active exploitation, data exposure | 24 hours | 7 days |
+| High — exploitable with effort | 48 hours | 14 days |
+| Medium / Low | 5 business days | Next release cycle |
+
+These are targets, not contractual SLAs. Stackbilt is a solo-founder operation and response times reflect that reality honestly. Critical issues affecting user data are prioritized above everything else.
+
+### Scope
+
+This policy covers all software published in this repository. For the full policy covering the entire Stackbilt-dev organization, see the [canonical security policy](https://docs.stackbilt.dev/security/).
+
+### Out of scope
+
+- Denial of service against free-tier services (Cloudflare handles DDoS)
+- Rate limiting bypass on non-authenticated endpoints (unless it enables data access)
+- Missing security headers on non-production deployments
+- Vulnerabilities in third-party dependencies where this repo is not the upstream maintainer
+
+### Disclosure
+
+- Stackbilt practices **coordinated disclosure** with a minimum 90-day window (30 days for critical).
+- Reporters are credited in release notes unless anonymity is requested.
+- Good-faith security research within this policy will not face legal action.
+
+### Contact
+
+- **Primary:** admin@stackbilt.dev
+- **Canonical policy:** https://docs.stackbilt.dev/security/