Add garbage-collect for home-manager

modules/home-manager/hm-expire.nix

@@ -0,0 +1,46 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: let
+  cfg = config.hm-expire;
+in {
+  options.hm-expire = {
+    enable = lib.mkEnableOption "Whether to enable hm-expire";
+    dates = lib.mkOption {
+      type = lib.types.str;
+      default = "weekly";
+    };
+    expire = lib.mkOption {
+      type = lib.types.str;
+      default = "-30 days";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # This creates a user-level systemd service
+    systemd.user.services.cleanup-home-manager = {
+      Unit = {
+        Description = "Cleanup old Home Manager generations";
+      };
+      Service = {
+        Type = "oneshot";
+        ExecStart = "${pkgs.bash}/bin/bash -c '${pkgs.home-manager}/bin/home-manager expire-generations \"${cfg.expire}\"'";
+      };
+    };
+
+    systemd.user.timers.cleanup-home-manager = {
+      Unit = {
+        Description = "Weekly cleanup of Home Manager generations";
+      };
+      Timer = {
+        OnCalendar = cfg.dates;
+        Persistent = true;
+      };
+      Install = {
+        WantedBy = ["timers.target"];
+      };
+    };
+  };
+}