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

package-manager.install

@@ -0,0 +1,11 @@
+post_install() {
+  /usr/lib/package-manager/init-nix.sh || true
+}
+
+post_upgrade() {
+  /usr/lib/package-manager/init-nix.sh || true
+}
+
+post_remove() {
+  echo ">>> package-manager removed. Nix itself was not removed."
+}