docs: add SECURITY.md, publish agentic security guide, remove openclaw guide
- Add SECURITY.md with vulnerability reporting policy
- Publish "The Shorthand Guide to Everything Agentic Security" with attack
vectors, sandboxing, sanitization, CVEs, and AgentShield coverage
- Add security guide to README guides section (3-column layout)
- Remove unpublished openclaw guide
- Copy security article images to assets/images/security/
Affaan Mustafa