feat(#2422): add Nix flake package and NixOS module

flake.nix

@@ -0,0 +1,88 @@
+{
+  description = "Hermes Web UI";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, ... }:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      linuxSystems = [ "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+      hermesModule = import ./nix/nixosModules.nix { inherit self; };
+      perSystem = forAllSystems (system: let
+        pkgs = import nixpkgs { inherit system; };
+        package = import ./nix/packages.nix {
+          inherit pkgs;
+          version = "0.51.0";
+        };
+        moduleChecks = if builtins.elem system linuxSystems then
+          let
+            moduleConfig = nixpkgs.lib.nixosSystem {
+              inherit system;
+              modules = [
+                hermesModule
+                {
+                  services.hermes-webui = {
+                    enable = true;
+                    package = package;
+                    host = "0.0.0.0";
+                    port = 8787;
+                    stateDir = "/var/lib/hermes-webui";
+                    agent.dir = "/var/lib/hermes-agent";
+                  };
+                }
+              ];
+            };
+            moduleServiceEnvironment = nixpkgs.lib.concatStringsSep "\n" moduleConfig.config.systemd.services.hermes-webui.serviceConfig.Environment;
+            envProbe = pkgs.writeText "hermes-webui-nixos-env-${system}.txt" moduleServiceEnvironment;
+          in
+          {
+            module-env-mapping = pkgs.runCommand "hermes-webui-nixos-module-${system}" {
+              nativeBuildInputs = [ pkgs.coreutils ];
+            } ''
+              grep -q 'HERMES_WEBUI_HOST=0.0.0.0' ${envProbe}
+              grep -q 'HERMES_WEBUI_PORT=8787' ${envProbe}
+              grep -q 'HERMES_WEBUI_STATE_DIR=/var/lib/hermes-webui' ${envProbe}
+              grep -q 'HERMES_WEBUI_AGENT_DIR=/var/lib/hermes-agent' ${envProbe}
+              touch "$out"
+            '';
+          }
+        else
+          { };
+      in
+      {
+        packages = {
+          hermes-webui = package;
+          default = package;
+        };
+
+        apps = {
+          default = {
+            type = "app";
+            program = "${package}/bin/hermes-webui";
+          };
+        };
+
+        checks = moduleChecks // {
+          package = package;
+        };
+      });
+    in
+    {
+      packages = forAllSystems (system: perSystem.${system}.packages);
+      apps = forAllSystems (system: perSystem.${system}.apps);
+      checks = nixpkgs.lib.genAttrs linuxSystems (system: perSystem.${system}.checks);
+
+      nixosModules = {
+        default = hermesModule;
+        hermes-webui = hermesModule;
+      };
+    };
+}