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/autostart/cachy-auto-update-notify.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=CachyOS Auto-Update notifications
+Name[de]=CachyOS Auto-Update Benachrichtigungen
+Comment=Delivers update notifications that arrived while nobody was logged in
+Comment[de]=Stellt Update-Benachrichtigungen zu, die eingingen, während niemand angemeldet war
+Exec=cachy-auto-update --deliver-notifications
+Icon=system-software-update
+Terminal=false
+NoDisplay=true
+X-GNOME-Autostart-enabled=true
+X-GNOME-Autostart-Delay=20
+X-KDE-autostart-after=panel