system: add nixos-rebuild-test helper script

Adds a helper script deployed to all hosts for testing feature branches.
Usage: nixos-rebuild-test <action> <branch>

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

system/nix.nix

@@ -1,5 +1,25 @@
-{ lib, ... }:
+{ lib, pkgs, ... }:
+let
+  nixos-rebuild-test = pkgs.writeShellApplication {
+    name = "nixos-rebuild-test";
+    runtimeInputs = [ pkgs.nixos-rebuild ];
+    text = ''
+      if [ $# -lt 2 ]; then
+        echo "Usage: nixos-rebuild-test <action> <branch>"
+        echo "Example: nixos-rebuild-test boot my-feature-branch"
+        exit 1
+      fi
+
+      action="$1"
+      branch="$2"
+      shift 2
+
+      exec nixos-rebuild "$action" --flake "git+https://git.t-juice.club/torjus/nixos-servers.git?ref=$branch" "$@"
+    '';
+  };
+in
 {
+  environment.systemPackages = [ nixos-rebuild-test ];
   nix = {
     gc = {
       automatic = true;