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>

hosts/miku-homelab/default.nix

@@ -12,6 +12,13 @@
 
   age.secrets.miku-homelab-wg.file = ../../secrets/miku-homelab-wg.age;
 
+  myPackages.extra = with pkgs; [
+    rocmPackages.rocm-smi
+    btrfs-progs
+    distrobox
+    steam-run
+  ];
+
   myNetworking = {
     hostName = "miku-homelab";
     useNetworkManager = true;

hosts/vps/default.nix

@@ -4,6 +4,7 @@
   imports = [
     ./hardware-configuration.nix
     ../../modules/profiles/server.nix
+    ../../modules/system/packages.nix
   ];
 
   # PLACEHOLDER: secrets/vps-wg.age does not exist yet. It has to be created

modules/system/packages.nix

@@ -1,14 +1,25 @@
-{pkgs, ... }: 
+{ lib, pkgs, config, ... }:
+
+let
+  cfg = config.myPackages;
+in
 {
+  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
-    rocmPackages.rocm-smi
-    btrfs-progs
-    distrobox
-    steam-run
-  ];
+      tmux
+    ] ++ cfg.extra;
+  };
 }