Add HOL Guard marketplace plugin

[kantorcodes]

Summary

• add hol-guard as a directly installable plugin in the agentskill.sh Claude Code marketplace
• source the maintained hashgraph-online/hol-guard-plugin repository rather than copying its runtime/skill assets
• document /plugin install hol-guard@agentskill-sh

HOL Guard itself is installed and operated by the referenced plugin. Its maintained plugin manifest and skill use the real hol-guard CLI for supported local harness protection, approvals/evidence, and the separate plugin-scanner CLI for package scanning. This contribution does not claim server-side or unsupported-harness interception.

Validation

• exact repository issue/PR/code dedupe for HOL Guard / hol-guard: 0 immediately before submission
• .claude-plugin/marketplace.json parses as JSON with one unique hol-guard entry
• the entry uses the same GitHub source-object shape as the current public agentskill.sh/marketplace.json
• canonical source repository contains .claude-plugin/plugin.json for hol-guard
• git diff --check passes

The diff is intentionally limited to the marketplace entry and its install documentation.