Add Darwin (macOS) support for comin

This PR adds full Darwin/macOS support to comin, allowing it to manage nix-darwin configurations in addition to NixOS systems.

What's Changed

Core Darwin Support

• Added runtime platform detection using runtime.GOOS to switch between Linux and Darwin implementations
• Implemented Darwin-specific service management using launchctl instead of systemctl
• Added Darwin machine ID detection using system_profiler to get the Hardware UUID
• Support for darwinConfigurations alongside existing nixosConfigurations

Darwin Module

• Created nix/darwin-module.nix that provides darwinModules.comin for nix-darwin users
• Uses launchd.daemons instead of systemd.services for service management
• Proper PATH configuration for launchd environment restrictions
• Automatic state directory setup and permissions

Darwin-Specific Activation

• Darwin uses activate and activate-user scripts instead of switch-to-configuration
• Platform-specific derivation paths and configuration evaluation
• Robust service restart mechanism to prevent hanging during self-updates

Service Restart Improvements

• Fixed the issue where comin would hang when trying to restart itself during Darwin activation
• Implemented a flag-based restart mechanism where comin exits cleanly after deployment and lets launchd restart it automatically
• No more arbitrary timeouts or race conditions

Path Fixes

• Used full paths for system commands (/usr/sbin/system_profiler, /bin/launchctl, etc.) to work with launchd's restricted PATH environment
• Fixed machine ID detection and service management commands

Usage

Darwin users can now add comin to their nix-darwin flake:

{
  inputs = {
    comin.url = "github:multivac61/comin";  # or upstream after merge
    # ... other inputs
  };

  outputs = { self, nix-darwin, comin, ... }: {
    darwinConfigurations."hostname" = nix-darwin.lib.darwinSystem {
      modules = [
        comin.darwinModules.comin  # Use darwinModules, not nixosModules
        {
          networking.hostName = "hostname";
          services.comin = {
            enable = true;
            remotes = [{
              name = "origin";
              url = "https://github.com/user/darwin-config.git";
              branches.main.name = "main";
            }];
          };
        }
      ];
    };
  };
}

Testing

I've been running it on a couple of aarch64-darwin computers running one of the later macOS versions.