Refine packaging layout and Arch build paths

* Move Arch-specific ignore rules into `packaging/arch/.gitignore` and simplify top-level `.gitignore`/`.dockerignore`.
* Update Arch `PKGBUILD` to sync from the project root and drop `packaging/` from the installed tree.
* Fix OS-specific `package.sh` helpers to resolve the new `packaging/*` locations correctly for Arch, Debian/Ubuntu, Fedora, and CentOS.

packaging/arch/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."
+}