Add cross-distribution OS package installers (Arch PKGBUILD, Debian control, RPM spec) and restructure tests.

Remove deprecated AUR and Ansible requirements installers.
Introduce Nix init + wrapper scripts and full packaging (Arch/DEB/RPM).
Associated conversation: https://chatgpt.com/share/693476a8-b9f0-800f-8e0c-ea5151295ce2

debian/postinst

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -e
+
+case "$1" in
+    configure)
+        if [ -x /usr/lib/package-manager/init-nix.sh ]; then
+            /usr/lib/package-manager/init-nix.sh || true
+        else
+            echo ">>> Warning: /usr/lib/package-manager/init-nix.sh not found or not executable."
+        fi
+        ;;
+esac
+
+exit 0