feat: add just command runner feature

[Copilot]

This PR adds support for the just command runner tool as a new devcontainer feature. Just is a handy way to save and run project-specific commands, serving as a modern alternative to make with simpler syntax.

What's Added

• Complete feature implementation following the repository's established patterns
• Version support with configurable version option (defaults to latest stable)
• Cross-architecture support for x86_64, i686, aarch64, and armv7
• Robust installation that handles GitHub API limitations gracefully
• Comprehensive testing including basic functionality and version-specific tests
• Full CI/CD integration in the existing test matrix

Usage

Users can now include the just feature in their devcontainer configuration:

{
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
    "features": {
        "ghcr.io/jsburckhardt/devcontainer-features/just:1": {}
    }
}

Or with a specific version:

{
    "features": {
        "ghcr.io/jsburckhardt/devcontainer-features/just:1": {
            "version": "1.42.0"
        }
    }
}

Implementation Details

The installation script downloads the appropriate binary from the official GitHub releases, extracts it, and installs it to /usr/local/bin. It includes proper error handling and fallback mechanisms for environments with limited GitHub API access.

Fixes #59.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/casey/just
  • Triggering command: curl -s REDACTED (http block)
• https://api.github.com/repos/casey/just/releases/latest
  • Triggering command: curl -s --max-time 10 REDACTED (http block)
• https://api.github.com/users/casey
  • Triggering command: curl -s REDACTED (http block)
• just.systems
  • Triggering command: curl -s --max-time 10 https://just.systems/install.sh (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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