Add Arch Linux support via standalone home-manager

[Copilot]

Enables using this NixOS flake on Arch Linux with Nix package manager and standalone home-manager. System management stays with Arch (pacman, systemd), user environment moves to Nix.

Core Changes

• flake.nix: Added homeConfigurations output with mkStandaloneHomeConfig helper for non-NixOS systems
• arch-hosts/example/home.nix: Reference configuration demonstrating module usage on Arch
• profiles/home-manager/base.nix: Reusable profile for standalone home-manager setups
• justfile-arch: Build commands adapted for home-manager (replaces nixos-rebuild with home-manager switch)

Usage

# After installing Nix on Arch
git checkout copilot/set-up-arch-with-nix
cp -r arch-hosts/example arch-hosts/$(hostname)
# Edit arch-hosts/$(hostname)/home.nix - set username, enable modules

# Add to flake.nix homeConfigurations:
"$USER@$(hostname)" = mkStandaloneHomeConfig {
  username = "$USER";
  stateVersion = "25.05";
  hostname = "$(hostname)";
  extraModules = [ ./arch-hosts/$(hostname)/home.nix ];
};

home-manager switch --flake .#$USER@$(hostname)

Architecture

Via Nix/home-manager: All home-manager modules (CLI tools, desktop configs, applications, dotfiles, theming)
Via Arch native: System services, kernel, hardware drivers, boot loader

Documentation

• ARCH-INDEX.md - Navigation hub
• ARCH-QUICKSTART.md - 5-minute setup
• ARCH-SETUP.md - Comprehensive guide with troubleshooting
• ARCH-CHECKLIST.md - Step-by-step installation
• NIXOS-TO-ARCH.md - Migration path from NixOS
• arch-hosts/README.md - Configuration patterns

Total: 3,006 lines of documentation covering installation, configuration, troubleshooting, and migration.

Original prompt

I want to move over to arch on my machine but i still want to use nix package manager for most of my files could you help me make a branch with changes needed to get it to work on a linux arch instance with home-managere and nix pkgmanager. there is most likely many changes needed so i want to do everything in a seperat arch branch

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