tweak: merge auto-upgrade.nix into automatic-upgrades.nix (gc 30d, optimise)

automatic-upgrades.nix

@@ -4,6 +4,8 @@
   # Enable automatic garbage collection (prevents /boot or / filling up)
   nix.gc.automatic     = true;
   nix.gc.dates         = "03:00";           # run GC at 03:00 daily
+  nix.gc.options       = "--delete-older-than 30d"; # drop generations older than 30 days
+  nix.optimise.automatic = true;            # hard-link identical store files
 
   # This host has no configuration.nix: point <nixos-config> at master.nix,
   # otherwise nixos-upgrade.service (and plain nixos-rebuild) cannot find the config.