feat: add NixOS module for nixos-options-mcp service

Module provides:
- services.nixos-options-mcp.enable - Enable the service
- services.nixos-options-mcp.package - Package to use
- services.nixos-options-mcp.database.type - sqlite or postgres
- services.nixos-options-mcp.database.name - SQLite filename
- services.nixos-options-mcp.database.connectionString - PostgreSQL URL
- services.nixos-options-mcp.indexOnStart - Revisions to index on start
- services.nixos-options-mcp.user/group - Service user/group
- services.nixos-options-mcp.dataDir - Data directory

Includes systemd hardening options.

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

flake.nix

@@ -7,8 +7,9 @@
 
   outputs = { self, nixpkgs }:
     let
+      lib = nixpkgs.lib;
       supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
-      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+      forAllSystems = lib.genAttrs supportedSystems;
       pkgsFor = system: nixpkgs.legacyPackages.${system};
     in
     {
@@ -67,5 +68,13 @@
             '';
           };
         });
+
+      nixosModules = {
+        nixos-options-mcp = { pkgs, ... }: {
+          imports = [ ./nix/module.nix ];
+          services.nixos-options-mcp.package = lib.mkDefault self.packages.${pkgs.system}.nixos-options;
+        };
+        default = self.nixosModules.nixos-options-mcp;
+      };
     };
 }