ci: add flake build + nixfmt checks on for-business PRs

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [for-business]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    name: nixfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@ef8a148080ab6020fd15196c2084a2eea5ff2d25 # v22
+
+      - name: Check formatting
+        run: nix run nixpkgs#nixfmt-rfc-style -- --check $(git ls-files '*.nix')
+
+  build:
+    name: build darwinConfiguration
+    runs-on: macos-26
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@ef8a148080ab6020fd15196c2084a2eea5ff2d25 # v22
+
+      - name: Build darwin system
+        run: nix build --print-build-logs '.#darwinConfigurations.default.system'