Drop pacman packaging, bakery-only distribution

bakery already fully covers what the PKGBUILD provided (binary, systemd
--user service where applicable, dependency declarations) except a LICENSE
copy, which bakery.toml's new license_file field now closes. Removes
packaging/arch/ and .forgejo/workflows/package.yml; adds the LICENSE
artifact to each release/dev-release/beta-release workflow's prepare
step. Not pacman-installed inside BOS today (BOS already consumes these
apps exclusively via build-local.sh's skel-staging), so this only removes
the option to `pacman -S` outside of BOS/bakery.

bakery.toml

@@ -4,6 +4,7 @@ binaries = ["breadbox", "breadbox-sync"]
 system_deps = ["gtk4", "gtk4-layer-shell", "librsvg"]
 optional_system_deps = ["hyprland"]
 bread_deps = []
+license_file = "LICENSE"
 
 [[service]]
 unit = "breadbox-sync.service"