Add Security section

[mpvasilis]

Adds a Security section. Vibe-coded apps frequently ship with exposed secrets, world-readable databases, or missing security headers, but the list had no section covering pre-deploy security. This fills that gap with three tools at different layers:

• Vibeproof — instant security scan for vibe-coded apps (URL or public repo), free, no signup.
• Snyk — code/dependency/IaC vulnerability scanning (free tier).
• Mozilla HTTP Observatory — free security-headers grading.

Added the matching Table of Contents entry. Formatting matches existing entries.

Disclosure: I'm involved with Vibeproof, but seeded the section with two well-established independent tools so it stands as a genuine category rather than a single product link. Happy to adjust.