Add AgentShield to Security section

[elliotllliu]

Hi! 👋

Adding AgentShield to the Security section.

AgentShield is a security scanner for AI agent skills, MCP servers, and plugins:

• 31 detection rules (prompt injection in 8 languages, data exfiltration, backdoors, tool poisoning)
• MCP runtime proxy for real-time interception
• Cross-file attack chain detection
• Free, offline, zero-config: npx @elliotllliu/agent-shield scan ./path/

MIT licensed. Thanks! 🙏