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.

.dockerignore

@@ -25,7 +25,5 @@ venv/
 .DS_Store
 Thumbs.db
 
-# Arch pkg artifacts
-*.pkg.tar.*
-*.log
-packaging/arch/package-manager-*
+# Logs
+*.log