refactor(packages): split core packages from per-host extras

modules/system/packages.nix now exposes myPackages.extra so hosts can
add their own tools on top of the always-installed core set, instead
of hardcoding every host's packages in one shared list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/system/packages.nix

@@ -1,14 +1,25 @@
-{pkgs, ... }: 
+{ lib, pkgs, config, ... }:
+
+let
+  cfg = config.myPackages;
+in
 {
-  environment.systemPackages = with pkgs; [
-    git
-    wget
-    curl
-    ethtool
-    fish
-    rocmPackages.rocm-smi
-    btrfs-progs
-    distrobox
-    steam-run
-  ];
+  options.myPackages = {
+    extra = lib.mkOption {
+      type = lib.types.listOf lib.types.package;
+      default = [ ];
+      description = "Extra packages to install on this host, beyond the core set.";
+    };
+  };
+
+  config = {
+    environment.systemPackages = with pkgs; [
+      git
+      wget
+      curl
+      ethtool
+      fish
+      tmux
+    ] ++ cfg.extra;
+  };
 }