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.

scripts/installation/arch/package.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 echo "[arch/package] Building Arch package (makepkg --nodeps)..."
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-PROJECT_ROOT="$(cd "${SCRIPT_DIR}/../.." && pwd)"
+PROJECT_ROOT="$(cd "${SCRIPT_DIR}/../../.." && pwd)"
 PKG_DIR="${PROJECT_ROOT}/packaging/arch"
 
 if [[ ! -f "${PKG_DIR}/PKGBUILD" ]]; then