chore(packaging): add AUR package sources for source, git, and binary builds

packaging/aur/ now holds the full content of three AUR repositories:
proton-drive-for-linux builds the latest tag from source,
proton-drive-for-linux-git builds main, and proton-drive-for-linux-bin
installs the binaries attached to the GitHub release. Each carries a
PKGBUILD, a generated .SRCINFO, an ignore-everything .gitignore, and the
0BSD license the AUR expects for package sources.

update.sh bumps the release-based packages and regenerates the .SRCINFO
files; push.sh publishes each package to its AUR remote. The README now
points users at the AUR packages and keeps the in-tree PKGBUILD as the
development build.

packaging/aur/push.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Publish the packages in this directory to their AUR repositories.
+#
+# Usage: ./push.sh [pkgbase ...]        (default: all three)
+#        MESSAGE="..." ./push.sh        (custom commit message)
+set -euo pipefail
+
+cd "$(dirname "$(readlink -f "$0")")"
+
+packages=("$@")
+if [[ ${#packages[@]} -eq 0 ]]; then
+  packages=(proton-drive-for-linux proton-drive-for-linux-git proton-drive-for-linux-bin)
+fi
+
+workdir="$(mktemp -d)"
+trap 'rm -rf "$workdir"' EXIT
+
+for pkg in "${packages[@]}"; do
+  test -f "$pkg/PKGBUILD" || { echo "no such package: $pkg" >&2; exit 1; }
+
+  # A fresh clone every time: the AUR repository, not this checkout, is what the
+  # commit history has to build on.
+  git -c init.defaultBranch=master clone -q "ssh://aur@aur.archlinux.org/$pkg.git" "$workdir/$pkg"
+  install -m644 "$pkg/PKGBUILD" "$pkg/.SRCINFO" "$pkg/.gitignore" "$pkg/LICENSE" "$workdir/$pkg/"
+
+  git -C "$workdir/$pkg" add -f PKGBUILD .SRCINFO .gitignore LICENSE
+  if git -C "$workdir/$pkg" diff --cached --quiet; then
+    echo "$pkg: no changes"
+    continue
+  fi
+
+  version="$(sed -n 's/^\tpkgver = //p' "$pkg/.SRCINFO" | head -n1)"
+  git -C "$workdir/$pkg" commit -q -m "${MESSAGE:-Update to $version}"
+  git -C "$workdir/$pkg" push -q origin HEAD:master
+  echo "$pkg: pushed ${MESSAGE:-$version}"
+done