Add Metain to AI tools list in README

[FiftyCFX]

Summary

Adding Metain (https://metain.dev) to the AI App Builders → Code-Generating section. Metain is an AI coding assistant purpose-built for Roblox Studio — it generates Luau scripts, UI, and animations from natural-language prompts through a browser chat that syncs live into Studio via an open-source plugin.

Type of Change

• New tool/resource
• Update existing entry
• Fix broken link
• Improve documentation
• Other (please describe)

Checklist

• I have read the CONTRIBUTING guidelines
• Tool/resource follows the awesome list format
• Links are working and tested
• Entry includes: Tool name, Pricing, Best For, Why It's Awesome
• No promotional/marketing language (focus on utility)
• Alphabetical order maintained (if applicable)

Additional Context

The list currently has no Roblox or Luau-specific vibe-coding tools. Metain fills that gap, similar workflow to Bolt or Lovable, but scoped to Roblox game development. Powered by Anthropic's Claude. The companion Studio plugin is open-source at github.com/metaindev/connect-plugin.

Entry was placed at the end of the Code-Generating table to match recent-addition ordering in that section. Happy to reposition if reviewers prefer strict alphabetical placement (would go after Marblism).