feat: Add templates for external configs

templates/system/.gitignore

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+result
+result-*
+.direnv/

templates/system/README.md

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+# External System Module Template
+
+This directory contains a template for creating external system configuration modules that can be referenced from the main `nixos-systems` repository.
+
+## Overview
+
+External modules allow you to maintain system configurations in separate Git repositories and reference them from the main `nixos-systems/inventory.nix` file using `builtins.fetchGit` or `builtins.fetchTarball`.
+
+## Usage
+
+### 1. Create Your Module Repository
+
+Copy `default.nix` from this template to your own Git repository. Customize it with your system configuration.
+
+### 2. Reference It in inventory.nix
+
+```nix
+{
+  "my-system-type" = {
+    devices = {
+      "hostname" = builtins.fetchGit {
+        url = "https://github.com/your-org/your-config-repo";
+        rev = "abc123def456...";  # Full commit hash for reproducibility
+        ref = "main";              # Optional: branch/tag name
+      };
+    };
+  };
+}
+```
+
+### 3. Module Structure
+
+Your `default.nix` must:
+- Accept `{ inputs, ... }` as parameters (you'll receive the same flake inputs)
+- Return a valid NixOS module with `{ config, lib, pkgs, ... }: { ... }`
+- Export configuration under the `config` attribute
+
+## Examples
+
+### Simple Configuration Module
+
+```nix
+{ inputs, ... }:
+
+{ config, lib, pkgs, ... }:
+
+{
+  config = {
+    time.timeZone = "America/New_York";
+    
+    environment.systemPackages = with pkgs; [
+      vim
+      git
+      htop
+    ];
+    
+    services.openssh.enable = true;
+  };
+}
+```
+
+### Advanced Module with Options
+
+```nix
+{ inputs, ... }:
+
+{ config, lib, pkgs, ... }:
+
+{
+  options.myorg.databaseUrl = lib.mkOption {
+    type = lib.types.str;
+    description = "Database connection URL";
+  };
+
+  config = {
+    # Use the option
+    services.postgresql = {
+      enable = true;
+      # ... configuration using config.myorg.databaseUrl
+    };
+  };
+}
+```
+
+## Benefits
+
+- **Separation of Concerns**: Keep specialized configurations in dedicated repositories
+- **Reusability**: Share configurations across multiple NixOS fleets
+- **Version Control**: Pin to specific commits for reproducibility
+- **Team Ownership**: Different teams can maintain their own config repos
+- **Security**: Private repositories for sensitive configurations
+
+## Integration with nixos-systems
+
+External modules are automatically integrated into the nixos-systems build:
+- They receive the same flake inputs (nixpkgs, home-manager, etc.)
+- They can use ugaif.* options if defined in the host type
+- They are merged with local overrides and base configuration
+- They work with all build methods (ISO, LXC, Proxmox, etc.)

templates/system/default.nix

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+{ inputs, ... }:
+
+# ============================================================================
+# External System Module Template
+# ============================================================================
+# This is a template for creating external system configuration modules
+# that can be referenced from nixos-systems/inventory.nix using builtins.fetchGit
+#
+# Usage in inventory.nix:
+#   "my-type" = {
+#     devices = {
+#       "hostname" = builtins.fetchGit {
+#         url = "https://github.com/your-org/your-config-repo";
+#         rev = "commit-hash";
+#       };
+#     };
+#   };
+#
+# This module will receive the same `inputs` flake inputs as the main
+# nixos-systems configuration, allowing you to use nixpkgs, home-manager, etc.
+
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  # ========== Module Options ==========
+  # Define any custom options your module needs
+  options = {
+    # Example: myorg.customOption = lib.mkOption { ... };
+  };
+
+  # ========== Module Configuration ==========
+  config = {
+    # Your system configuration goes here
+    # This can include any NixOS options
+
+    # Example: Set timezone
+    # time.timeZone = "America/New_York";
+
+    # Example: Install packages
+    # environment.systemPackages = with pkgs; [
+    #   vim
+    #   git
+    # ];
+
+    # Example: Configure services
+    # services.openssh.enable = true;
+
+    # Example: Use ugaif options if available from nixos-systems
+    # ugaif.users.myuser.enable = true;
+  };
+}