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/logrotate/cachy-auto-update

@@ -0,0 +1,9 @@
+/var/log/cachy-auto-update/*.log {
+	weekly
+	rotate 8
+	missingok
+	notifempty
+	compress
+	delaycompress
+	create 0644 root root
+}