Add guard-system plugin

[PaoloC68]

Summary

Adds the Guard System plugin to the Agent Zero Plugin Index.

• Plugin repo: https://github.com/PaoloC68/a0-guard-system
• What it does: Security guards for tool execution and prompt safety. Blocks tools with flagged scan status and detects prompt injection patterns.
• Features:
  • Tool blocking via RepairableException in tool_execute_before extension point
  • Prompt length limiting and injection pattern detection in message_loop_prompts_after
  • Configurable injection patterns (regex supported)
  • Per-project and per-agent-profile settings
  • Settings UI with framework CSS classes
• Tags: security, tools, prompting