Add HOL Guard security skill

[kantorcodes]

Summary

• add the HOL Guard Agent Skill under skills/hol-guard
• register hol-guard in the repository's Claude Code marketplace
• point users to the canonical HOL Guard project and product documentation

HOL Guard is a local security layer for AI coding harnesses. The skill installs and invokes hol-guard for Guard-owned harness protection, approvals, receipts, and evidence, and uses the separately distributed plugin-scanner for plugin, skill, MCP, and marketplace verification.

Install / discovery

The repository's existing skills.sh-compatible flow can install the added skill directly:

npx skills add ReScienceLab/opc-skills --skill hol-guard

The marketplace entry also exposes the same local skill at ./skills/hol-guard for Claude Code marketplace discovery.

Validation

• fresh upstream issue/PR search found no existing HOL Guard contribution
• .claude-plugin/marketplace.json remains valid JSON and references the committed skill path
• skill frontmatter includes the repository-supported name, description, and license fields
• install checking was made platform-neutral rather than relying on POSIX-only command -v
• no generic framework/API changes or unrelated files

Canonical source: https://github.com/hashgraph-online/hol-guard-plugin
Product docs: https://hol.org/guard