feat: add Nix flake packaging and install docs

Enable nix run/profile install for git-open and document the workflow for Nix users.

Author: zhaochunqi

.gitignore

@@ -26,3 +26,6 @@ dist/
 
 # windsurf rules
 .windsurfrules
+
+# Nix build result symlink
+result

README.md

@@ -57,6 +57,32 @@ chmod +x git-open
 sudo mv git-open /usr/local/bin/
 ```
 
+
+### Nix
+
+Using flakes (recommended):
+
+```sh
+# one-shot run without installing
+nix run github:zhaochunqi/git-open
+
+# install into your profile
+nix profile install github:zhaochunqi/git-open
+
+# or pin a release tag
+nix profile install github:zhaochunqi/git-open/v2.4.2
+```
+
+From a local checkout:
+
+```sh
+nix build
+./result/bin/git-open version
+
+# development shell with Go toolchain
+nix develop
+```
+
 ### mise with ubi
 
 For users who prefer mise for version management:

flake.lock

@@ -0,0 +1,93 @@
+{
+  description = "Open the web URL of the current Git repository in a browser";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+
+      # Keep in sync with the latest release tag when packaging.
+      packageVersion = "2.4.2";
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          commit = self.shortRev or self.dirtyShortRev or "dirty";
+        in
+        {
+          default = self.packages.${system}.git-open;
+          git-open = pkgs.buildGoModule {
+            pname = "git-open";
+            version = packageVersion;
+            src = self;
+
+            # Bump this after changing go.mod / go.sum:
+            #   nix build 2>&1 | rg 'got:'
+            vendorHash = "sha256-WaM79VI/0eSCj4t6Myji/WPrjRSUFfaQjHfqPXGULgM=";
+
+            # Needed by worktree-related tests in checkPhase.
+            nativeBuildInputs = [ pkgs.git ];
+
+            ldflags = [
+              "-s"
+              "-w"
+              "-X github.com/zhaochunqi/git-open/cmd.Version=${packageVersion}"
+              "-X github.com/zhaochunqi/git-open/cmd.CommitHash=${commit}"
+              "-X github.com/zhaochunqi/git-open/cmd.BuildDate=unknown"
+            ];
+
+            # ldflags inject release version, so skip tests that assert the
+            # default "dev" values from source.
+            checkFlags = [ "-skip=^Test_rootCmd_VersionFlag$" ];
+
+            meta = with pkgs.lib; {
+              description = "Open the web URL of the Git repository";
+              homepage = "https://github.com/zhaochunqi/git-open";
+              changelog = "https://github.com/zhaochunqi/git-open/releases/tag/v${packageVersion}";
+              license = licenses.mit;
+              mainProgram = "git-open";
+              platforms = platforms.unix ++ platforms.windows;
+            };
+          };
+        }
+      );
+
+      apps = forAllSystems (system: {
+        default = {
+          type = "app";
+          program = "${self.packages.${system}.git-open}/bin/git-open";
+        };
+      });
+
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              go
+              gopls
+              gotools
+              git
+            ];
+          };
+        }
+      );
+
+      formatter = forAllSystems (system: nixpkgs.legacyPackages.${system}.nixfmt-rfc-style);
+    };
+}