add aspects/common/ NixOS modules

Migrates common NixOS configuration to dendritic aspect pattern:
boot, console, firewall, locale, nix, openssh, programs, security,
services, tailscale, users

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

aspects/common/nix.nix

@@ -0,0 +1,39 @@
+{ ... }:
+{
+  flake.modules.nixos.common-nix = { inputs, ... }: {
+    imports = [ inputs.nixos-cli.nixosModules.nixos-cli ];
+
+    nix = {
+      settings = {
+        auto-optimise-store = true;
+        connect-timeout = 10;
+        log-lines = 25;
+        min-free = 128000000;
+        max-free = 1000000000;
+        trusted-users = [ "@wheel" ];
+      };
+      extraOptions = "experimental-features = nix-command flakes";
+      gc = {
+        automatic = true;
+        options = "--delete-older-than 8d";
+      };
+    };
+
+    nixpkgs.config = {
+      allowUnfree = true;
+      enableParallelBuilding = true;
+      buildManPages = false;
+      buildDocs = false;
+    };
+
+    services.nixos-cli = {
+      enable = true;
+      config = {
+        use_nvd = true;
+        ignore_dirty_tree = true;
+      };
+    };
+
+    system.stateVersion = "22.11";
+  };
+}