feat(plugins): add HOL Guard Plugin

[kantorcodes]

Summary

• add HOL Guard Plugin to the Development & Workflow README list
• add the mirrored installable bundle under plugins/hashgraph-online/hol-guard-plugin
• add marketplace and plugins.json metadata with the official HOL icon as the plugin icon
• ignore local .DS_Store files

Verification

• npm test (plugins/hashgraph-online/hol-guard-plugin)
• jq empty marketplace, plugins.json, and plugin manifests
• jq checks for marketplace icon path and plugin entry
• uv run python scripts/check-alphabetical.py README.md
• uv run python scripts/validate-plugin-pr.py --base-ref origin/main
• node build.js
• git diff --check

Upstream prerequisite

• HOL Guard Plugin official icon source PR merged: fix(plugin): use official HOL icon hol-guard-plugin#4

[gemini-code-assist]

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