refactor: split base.nix into core.nix + base.nix layers

core.nix is the new minimal layer: user, SSH, fish, nix flakes/gc,
locale, and a handful of essential packages. Suitable for any headless
host that just needs to be SSHable and manageable.

base.nix now imports core.nix and adds the day-to-day quality-of-life
layer: avahi, mosh, direnv, nix-ld, earlyoom, zramSwap, CLI tools,
and build toolchains. All existing hosts that import base.nix are
unchanged.

bootstrap is updated to import core.nix directly instead of
duplicating the config inline.

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

hosts/modules/core.nix

@@ -0,0 +1,67 @@
+{ config, pkgs, inputs, ... }:
+{
+  users = {
+    users.alex = {
+      isNormalUser = true;
+      description = "Alexander Wainwright";
+      extraGroups = [
+        "dialout"
+        "input"
+        "networkmanager"
+        "plugdev"
+        "render"
+        "video"
+        "wheel"
+      ];
+      packages = [];
+      shell = pkgs.fish;
+    };
+    groups.pluggdev = {};
+  };
+
+  programs.fish.enable = true;
+
+  documentation.man.generateCaches = false;
+
+  time.timeZone = "Australia/Brisbane";
+
+  i18n.defaultLocale = "en_GB.UTF-8";
+  i18n.extraLocaleSettings = {
+    LC_ADDRESS = "en_AU.UTF-8";
+    LC_IDENTIFICATION = "en_AU.UTF-8";
+    LC_MEASUREMENT = "en_AU.UTF-8";
+    LC_MONETARY = "en_AU.UTF-8";
+    LC_NAME = "en_AU.UTF-8";
+    LC_NUMERIC = "en_AU.UTF-8";
+    LC_PAPER = "en_AU.UTF-8";
+    LC_TELEPHONE = "en_AU.UTF-8";
+    LC_TIME = "en_AU.UTF-8";
+  };
+
+  console.keyMap = "dvorak";
+
+  nixpkgs.config.allowUnfree = true;
+
+  nix.settings = {
+    experimental-features = [ "nix-command" "flakes" ];
+    auto-optimise-store = true;
+    trusted-users = [ "root" "alex" ];
+  };
+
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 30d";
+  };
+
+  services.openssh.enable = true;
+
+  environment.variables.EDITOR = "nvim";
+
+  environment.systemPackages = with pkgs; [
+    curl
+    git
+    neovim
+    wget
+  ];
+}