Add Runtype Skills plugin

[bookernath]

Summary

• Adds Runtype Skills to the Plugins section
• It's a .codex-plugin format plugin (cross-compatible with both Codex CLI and Claude Code) with agent skills for building, deploying, and operating AI products on Runtype's managed edge runtime
• MIT licensed, published by runtypelabs
• Includes 6 skills: runtype, runtype-admin, runtype-build-product, runtype-persona, runtype-sdk-marathon, runtype-templates

Checklist per CONTRIBUTING.md

• Directly related to Codex CLI (.codex-plugin/plugin.json format plugin)
• Actively maintained (recent commits)
• Clear one-sentence description explaining value
• Star badge included (flat-square style)
• Added to most specific category (Plugins)
• No duplicate — nothing similar exists in the list
• Free/open-source (MIT license)