feat(base): add nix store garbage collection and boot entry limits

Configure automatic weekly GC across all platforms with
--delete-older-than 30d. Limits systemd-boot entries to 10 on NixOS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

modules/base/default.nix

@@ -1,6 +1,7 @@
 # Base configuration shared across all systems
 {
   imports = [
+    ./gc.nix
     ./nix-config.nix
     ./unfree-packages.nix
   ];

modules/base/gc.nix

@@ -0,0 +1,20 @@
+# Nix store garbage collection and boot entry limits
+{
+  lib,
+  pkgs,
+  ...
+}: {
+  nix.gc =
+    {
+      automatic = true;
+      options = "--delete-older-than 30d";
+    }
+    // lib.optionalAttrs pkgs.stdenv.isLinux {
+      dates = "weekly";
+    }
+    // lib.optionalAttrs pkgs.stdenv.isDarwin {
+      interval = {Weekday = 0;};
+    };
+
+  boot.loader.systemd-boot.configurationLimit = lib.mkIf pkgs.stdenv.isLinux 10;
+}