feat: add NixOS module for declarative deployment

Adds nixos-module.nix with services.oubliette options (enable, package,
settings, configFile) and a hardened systemd service. Exposes the module
as nixosModules.default in flake.nix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos-module.nix

@@ -0,0 +1,64 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.oubliette;
+  tomlFormat = pkgs.formats.toml { };
+
+  configFile =
+    if cfg.configFile != null
+    then cfg.configFile
+    else tomlFormat.generate "oubliette.toml" cfg.settings;
+in
+{
+  options.services.oubliette = {
+    enable = lib.mkEnableOption "Oubliette SSH honeypot";
+
+    package = lib.mkPackageOption pkgs "oubliette" { };
+
+    configFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      default = null;
+      description = ''
+        Path to a pre-written TOML configuration file.
+        Mutually exclusive with {option}`settings`.
+      '';
+    };
+
+    settings = lib.mkOption {
+      type = tomlFormat.type;
+      default = { };
+      description = ''
+        Configuration for Oubliette as a Nix attribute set.
+        Will be converted to TOML. Mutually exclusive with {option}`configFile`.
+        See oubliette.toml.example for available options.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = !(cfg.configFile != null && cfg.settings != { });
+        message = "services.oubliette: `configFile` and `settings` are mutually exclusive.";
+      }
+    ];
+
+    services.oubliette.settings.ssh.host_key_path = lib.mkDefault "/var/lib/oubliette/host_key";
+
+    systemd.services.oubliette = {
+      description = "Oubliette SSH Honeypot";
+      after = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+
+      serviceConfig = {
+        ExecStart = "${lib.getExe cfg.package} -config ${configFile}";
+        DynamicUser = true;
+        StateDirectory = "oubliette";
+        NoNewPrivileges = true;
+        ProtectSystem = "strict";
+        ProtectHome = true;
+        ReadWritePaths = [ "/var/lib/oubliette" ];
+      };
+    };
+  };
+}