feat(skills): add HOL Guard security skill - #176

Summary

Adds a focused Agent Skill for the third-party HOL Guard CLI to the Gemini CLI Security extension.

The skill covers:

• installing and invoking hol-guard for supported-harness protection
• treating deny, review-required, and Guard error states as stop conditions
• approval/evidence guidance
• plugin-scanner inspection for agent plugins, skills, and MCP packages

Architecture boundary

This intentionally does not claim that Gemini CLI exposes a native HOL Guard pre-tool interception hook. Runtime blocking remains limited to HOL Guard's currently supported harness integrations; package scanning is inspection rather than runtime enforcement.

Validation

• followed the repository's existing single-file skills/<name>/SKILL.md convention
• verified the extension manifest does not require explicit per-skill registration
• exact upstream searches for HOL Guard and hol-guard across issues/PRs and code were clean before submission
• re-read the committed skill from the fork branch before opening this PR

HOL Guard source: https://github.com/hashgraph-online/hol-guard-plugin