feat(nix): convert git env script to nix shell app

Signed-off-by: Cameron Smith <cameron.ray.smith@gmail.com>

Author: cameronraysmith

nix/modules/git-env.nix

@@ -0,0 +1,64 @@
+{
+  inputs,
+  lib,
+  config,
+  pkgs,
+  ...
+}:
+
+{
+  perSystem =
+    { system, pkgs, ... }:
+    {
+      packages.set-git-env = pkgs.writeShellApplication {
+        name = "set-git-env";
+        runtimeInputs = with pkgs; [
+          git
+          coreutils
+          gnused
+          gnugrep
+        ];
+        text = ''
+          #!/usr/bin/env bash
+
+          set -euo pipefail
+
+          update_or_append() {
+              local var_name=$1
+              local var_value=$2
+              local file=".env"
+
+              # Create .env file if it doesn't exist
+              if [ ! -f "$file" ]; then
+                  touch "$file"
+              fi
+              
+              if [ -s "$file" ] && [ "$(tail -c1 "$file"; echo x)" != $'\nx' ]; then
+                  echo >> "$file"
+              fi
+
+              if grep -q "^$var_name=" "$file"; then
+                  # Use sed with backup extension for compatibility
+                  sed -i.bak "s/^$var_name=.*/$var_name=$var_value/" "$file"
+                  # Remove backup file
+                  rm -f "$file.bak"
+              else
+                  echo "$var_name=$var_value" >> "$file"
+              fi
+          }
+
+          GIT_REPO_NAME=$(basename -s .git "$(git config --get remote.origin.url 2>/dev/null || echo "unknown-repo")")
+          update_or_append "GIT_REPO_NAME" "$GIT_REPO_NAME"
+
+          GIT_REF=$(git symbolic-ref -q --short HEAD 2>/dev/null || git rev-parse HEAD 2>/dev/null || echo "unknown-ref")
+          update_or_append "GIT_REF" "$GIT_REF"
+
+          GIT_SHA=$(git rev-parse HEAD 2>/dev/null || echo "unknown-sha")
+          update_or_append "GIT_SHA" "$GIT_SHA"
+
+          GIT_SHA_SHORT=$(git rev-parse --short HEAD 2>/dev/null || echo "unknown")
+          update_or_append "GIT_SHA_SHORT" "$GIT_SHA_SHORT"
+        '';
+      };
+    };
+}