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

scripts/pkgmgr-wrapper.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Enable flakes if not already configured.
+if [[ -z "${NIX_CONFIG:-}" ]]; then
+  export NIX_CONFIG="experimental-features = nix-command flakes"
+fi
+
+# Run Kevin’s package manager via Nix flake
+exec nix run "github:kevinveenbirkenbach/package-manager#pkgmgr" -- "$@"