poppy: add weekly nix GC launchd agent (delete-older-than 14d)

nix.gc.automatic requires nix.enable=true, but poppy uses the
Determinate nix installer (nix.enable=false). Define a launchd
user agent directly that runs nix-collect-garbage --delete-older-than 14d
weekly at Sun 03:00.

hosts/poppy/default.nix

@@ -20,6 +20,25 @@
   nixpkgs.config.allowUnfree = true;
   nix.enable = false;
 
+  # Автоматический сбор мусора: удалять пути старше 14 дней, по ночам в 3:00.
+  # nix.gc не работает с nix.enable=false, поэтому launchd-агент напрямую.
+  launchd.user.agents.nix-gc = {
+    serviceConfig = {
+      Label = "local.nix.gc";
+      ProgramArguments = [
+        "/nix/var/nix/profiles/default/bin/nix-collect-garbage"
+        "--delete-older-than"
+        "14d"
+      ];
+      StartCalendarInterval = {
+        Weekday = 0;
+        Hour = 3;
+      };
+      StandardOutPath = "/tmp/nix-gc.log";
+      StandardErrorPath = "/tmp/nix-gc.log";
+    };
+  };
+
   users.users.mikl = {
     home = "/Users/mikl";
     shell = pkgs.zsh;