Add beast-dev-guard plugin

[thebeastagi]

What this PR does

Adds beast-dev-guard, a docs-only plugin for verification-first agentic development.

• Plugin name: beast-dev-guard
• Type: remote source
• Source URL + pinned SHA (remote), or vendored path (local): https://github.com/thebeastagi/beast-dev-guard.git at df33477dfa730a660851f3243f42d943a36486e2
• Homepage: https://github.com/thebeastagi/beast-dev-guard

Ownership

• I own this plugin or have the right to distribute it.
• The source repo is published under our official org (or I've explained why not below).

The plugin is original content from The Beast (@thebeastagi) and is published under the MIT License.

Checklist

• Added/updated exactly one entry in .grok-plugin/marketplace.json (valid JSON, kebab-case name).
• Remote source pins a full 40-char lowercase commit sha, and that commit is public + reachable.
• Regenerated .grok-plugin/plugin-index.json (python3 scripts/generate-plugin-index.py).
• python3 scripts/validate-catalog.py passes locally.
• python3 scripts/generate-plugin-index.py --check passes locally.
• homepage + clear description set; local plugins include README.md + .grok-plugin/plugin.json.
• License is stated.

Security

• No curl | bash, remote-code download/exec, or postinstall RCE.
• No reading/exfiltration of secrets, tokens, .env, or env vars.
• Hooks and MCP scope are least-privilege.
• Network endpoints this plugin calls (and why): None.
• Credentials/permissions it requires (and why): None.

Docs-only plugin — no scripts, hooks, MCP/LSP servers; calls no network endpoints; requires no credentials or permissions.

Notes for reviewers

The source is published from our official organization and all plugin content is original. The remote commit is immutable-pinned, publicly reachable, and the catalog/index validation commands pass locally.