Add Artix Linux support (bootstrap + chroot setup)

[SkidProxyForker]

Motivation

• Add support for Artix Linux as an alternative rootfs target so users can build images with a non-systemd distribution on x86_64 and ARM64 devices.
• Use the upstream gripped/artix-bootstrap project to reliably bootstrap Artix rootfs instances rather than implementing a custom bootstrapper.
• Provide an automated chroot setup that configures pacman, installs desktop packages, enables OpenRC services, and provisions a user account for a complete image build.

Description

• Documented Artix support and usage in README.md and added example invocation sudo ./build_complete.sh dedede distro=artix.
• Extended build_complete.sh to accept distro=artix and set a sensible default release (rolling) when building Artix images.
• Updated build_rootfs.sh to add git to declared dependencies, detect distro=artix, clone https://github.com/gripped/artix-bootstrap.git, locate and execute the upstream bootstrap script, and then hand off to a chroot setup script at /opt/setup_rootfs_artix.sh.
• Added rootfs/opt/setup_rootfs_artix.sh which runs inside the chroot to configure pacman mirrors and keys, install desktop or custom packages, enable OpenRC services (including a kill-frecon script), create the unprivileged user, manage passwords, and enable the shimboot greeter.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task