Add cachy-auto-update: unattended background updates for CachyOS

A root systemd service applies pacman, AUR, Flatpak and AppImage updates on
its own, gated on battery state, gaming activity and whether anybody else is
using the package system. The CLI is deliberately two switches plus status.

No user password is stored anywhere: pacman runs as root directly, and the AUR
step - which makepkg forbids running as root - drops to a locked system account
that sudoers permits to call pacman without a password.

res/sysusers/cachy-auto-update.conf

@@ -0,0 +1,6 @@
+# The account that builds and installs AUR packages.
+#
+# makepkg, paru and yay all refuse to run as root, so the update service drops
+# to this account for the AUR step. It has no password and no shell: nobody can
+# log into it, and only root can become it.
+u cachy-auto-update - "CachyOS Auto-Update builder" /var/lib/cachy-auto-update/builder /usr/bin/nologin