feat(nix): add nixpkgs-search-mcp NixOS module

Add NixOS module for deploying nixpkgs-search as systemd services:
- Runs separate MCP servers for options (port 8082) and packages (port 8083)
- Shared database configuration (SQLite or PostgreSQL)
- Separate indexing service that runs before servers start
- options.enable and packages.enable flags (both default to true)
- indexFlags option for customizing index command (--no-packages, etc.)

Also update flake.nix:
- Register new module as nixpkgs-search-mcp
- Set as default nixosModule

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

flake.nix

@@ -61,6 +61,10 @@
         });
 
       nixosModules = {
+        nixpkgs-search-mcp = { pkgs, ... }: {
+          imports = [ ./nix/nixpkgs-search-module.nix ];
+          services.nixpkgs-search.package = lib.mkDefault self.packages.${pkgs.system}.nixpkgs-search;
+        };
         nixos-options-mcp = { pkgs, ... }: {
           imports = [ ./nix/module.nix ];
           services.nixos-options-mcp.package = lib.mkDefault self.packages.${pkgs.system}.nixos-options;
@@ -69,7 +73,7 @@
           imports = [ ./nix/hm-options-module.nix ];
           services.hm-options-mcp.package = lib.mkDefault self.packages.${pkgs.system}.hm-options;
         };
-        default = self.nixosModules.nixos-options-mcp;
+        default = self.nixosModules.nixpkgs-search-mcp;
       };
     };
 }