Publish Arch Linux package (AUR)

[rapatao]

What

Publish an AUR (Arch User Repository) package so Arch/Manjaro users can
yay -S md2 / paru -S md2.

Why

Rounds out Linux distro coverage alongside #28 (deb/rpm/apk). Arch doesn't
use nfpm-style packages — AUR wants a PKGBUILD in a git repo pushed to
aur.archlinux.org, similar in spirit to how the Homebrew tap and NUR
package are handled today (see homebrew_casks:/nix: sections in
.goreleaser.yaml).

Approach

GoReleaser has a built-in aurs: publisher (binary release, no build-from-source
needed since md2 has no CGO deps):

1. Add aurs: section to .goreleaser.yaml, pointing at a ssh://aur@aur.archlinux.org/md2-bin.git
   repo, private key via AUR_KEY secret (goreleaser docs: https://goreleaser.com/customization/aur/).
2. Requires registering the md2-bin package name on AUR first (one-time,
   manual, needs an Arch Linux account) before CI can push to it.
3. Mirror homepage/description/license from the existing
   homebrew_casks/nix sections for consistency.
4. Update README install section with the AUR helper command.