Add Matilda Code Features to collection index

[maincode-dave]

Summary

Adds the Maincode dev container Feature collection to the public collection index so it is discoverable in VS Code Dev Containers, GitHub Codespaces, and on https://containers.dev/features.

• Collection name: Matilda Code Features
• Maintainer: Maincode
• Repository: https://github.com/MaincodeHQ/devcontainer-features
• OCI reference: ghcr.io/maincodehq/devcontainer-features/matilda-code

What the collection provides

The collection currently publishes the matilda-code Feature, which installs the Matilda Code CLI (@maincode-ai/matilda-code) — Maincode's terminal-based AI coding agent. It provisions Node.js >= 22 automatically when the base image does not provide it, then installs the CLI globally.

The Feature is already published to GHCR under the ociReference above (tags 1, 1.0, 1.0.0, latest), and the source repository is public.

Checklist

• Entry follows the existing name / maintainer / contact / repository / ociReference schema used by all other entries.
• Referenced OCI artifact is publicly readable at ghcr.io/maincodehq/devcontainer-features/matilda-code.
• Referenced source repository is public.