feat(nixos): add CrateDocs MCP service module

- Introduced a NixOS module for configuring the CrateDocs MCP server.
- Added options for enabling the service, setting the port, user, and group.
- Configured a hardened systemd service with security restrictions.

config.nix

@@ -0,0 +1,67 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.services.cratedocs-mcp;
+in
+{
+  options.services.cratedocs-mcp = {
+    enable = lib.mkEnableOption "CrateDocs MCP server";
+    
+    port = lib.mkOption {
+      type = lib.types.int;
+      default = 3000;
+      description = "Port to listen on for HTTP/SSE server";
+    };
+    
+    user = lib.mkOption {
+      type = lib.types.str;
+      default = "cratedocs-mcp";
+      description = "User to run the service as";
+    };
+    
+    group = lib.mkOption {
+      type = lib.types.str;
+      default = "cratedocs-mcp";
+      description = "Group to run the service as";
+    };
+  };
+  
+  config = lib.mkIf cfg.enable {
+    systemd.services.cratedocs-mcp = {
+      description = "CrateDocs MCP server";
+      after = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+      
+      serviceConfig = {
+        ExecStart = "${lib.getExe pkgs.cratedocs-mcp} --port ${toString cfg.port}";
+        Restart = "always";
+        User = cfg.user;
+        Group = cfg.group;
+        DynamicUser = true;
+        StateDirectory = "cratedocs-mcp";
+        CacheDirectory = "cratedocs-mcp";
+        
+        # Security hardening
+        PrivateTmp = true;
+        ProtectSystem = "strict";
+        ProtectHome = true;
+        NoNewPrivileges = true;
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        RemoveIPC = true;
+        PrivateMounts = true;
+        ProtectHostname = true;
+        ProtectClock = true;
+        ProtectKernelTunables = true;
+        ProtectKernelModules = true;
+        ProtectKernelLogs = true;
+        ProtectControlGroups = true;
+        LockPersonality = true;
+        MemoryDenyWriteExecute = true;
+        RestrictAddressFamilies = [ "AF_INET" "AF_INET6" ];
+        SystemCallFilter = "@system-service";
+        SystemCallErrorNumber = "EPERM";
+      };
+    };
+  };
+}

flake.nix

@@ -42,5 +42,6 @@
             default = cratedocs-mcp;
           };
         };
+      nixosModules.default = import ./config.nix;
     };
 }