feat: add Nix flake for declarative builds and development

.gitignore

@@ -148,3 +148,7 @@ CLAUDE.md
 
 # Pnpm
 .pnpm-store/
+
+# Nix
+result
+.direnv/

README.md

@@ -144,6 +144,21 @@ Verify installation:
 openspec --version
 ```
 
+<details>
+<summary><strong>Alternative: Install with Nix</strong> (click to expand)</summary>
+
+If you use [Nix](https://nixos.org/), you can run OpenSpec directly without global installation:
+
+```bash
+# Run directly from the repository
+nix run github:Fission-AI/OpenSpec -- --version
+
+# Or enter a development shell
+nix develop github:Fission-AI/OpenSpec
+```
+
+</details>
+
 #### Step 2: Initialize OpenSpec in your project
 
 Navigate to your project directory:
@@ -376,6 +391,8 @@ Run `openspec update` whenever someone switches tools so your agents pick up the
 - Develop CLI locally: `pnpm run dev` or `pnpm run dev:cli`
 - Conventional commits (one-line): `type(scope): subject`
 
+**Nix users:** Run `nix develop` to enter a development shell with Node.js and pnpm pre-configured.
+
 ## License
 
 MIT

flake.nix

@@ -0,0 +1,82 @@
+{
+  description = "OpenSpec - AI-native system for spec-driven development";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        nodejs = pkgs.nodejs_22;
+        packageJson = pkgs.lib.importJSON ./package.json;
+      in
+      {
+        packages.default = pkgs.stdenv.mkDerivation (finalAttrs: {
+          pname = "openspec";
+          version = packageJson.version;
+
+          src = ./.;
+
+          nativeBuildInputs = [
+            nodejs
+            pkgs.pnpm
+            pkgs.pnpmConfigHook
+            pkgs.makeWrapper
+          ];
+
+          # When pnpm-lock.yaml changes, update hash by running:
+          # nix build 2>&1 | grep 'got:' | awk '{print $2}'
+          pnpmDeps = pkgs.fetchPnpmDeps {
+            inherit (finalAttrs) pname version src;
+            hash = "sha256-vAlqVFaBN7KMlyP4HKbsMkaYrA5Yf2l5a+PLCZ6KOzs=";
+            fetcherVersion = 3;
+          };
+
+          buildPhase = ''
+            runHook preBuild
+            pnpm run build
+            runHook postBuild
+          '';
+
+          installPhase = ''
+            runHook preInstall
+
+            mkdir -p $out/lib/openspec
+            cp -r dist $out/lib/openspec/
+            cp -r bin $out/lib/openspec/
+            cp -r node_modules $out/lib/openspec/
+            cp package.json $out/lib/openspec/
+
+            mkdir -p $out/bin
+            makeWrapper ${nodejs}/bin/node $out/bin/openspec \
+              --add-flags "$out/lib/openspec/bin/openspec.js"
+
+            runHook postInstall
+          '';
+
+          meta = with pkgs.lib; {
+            description = "AI-native system for spec-driven development";
+            homepage = "https://github.com/Fission-AI/OpenSpec";
+            license = licenses.mit;
+            maintainers = [ ];
+            mainProgram = "openspec";
+            platforms = platforms.all;
+          };
+        });
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            nodejs
+            pkgs.pnpm
+          ];
+        };
+
+        apps.default = flake-utils.lib.mkApp {
+          drv = self.packages.${system}.default;
+        };
+      }
+    );
+}