security: apply Agents Council recommendations

- Add USER node directive to Dockerfile for non-root container execution
- Update SECURITY.md with Node.js version requirements (CVE-2025-59466, CVE-2026-21636)
- Add Docker security best practices documentation
- Document detect-secrets usage for local security scanning

Reviewed-by: Agents Council (5/5 approval)
Security-Score: 8.8/10
Watchdog-Verdict: SAFE WITH CONDITIONS

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Dockerfile

@@ -32,4 +32,9 @@ RUN pnpm ui:build
 
 ENV NODE_ENV=production
 
+# Security hardening: Run as non-root user
+# The node:22-bookworm image includes a 'node' user (uid 1000)
+# This reduces the attack surface by preventing container escape via root privileges
+USER node
+
 CMD ["node", "dist/index.js"]