ci: add lint + flake check workflow

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@v13
+      - name: Check formatting (nixfmt)
+        run: nix run nixpkgs#nixfmt-rfc-style -- --check *.nix docs/*.nix tests/*.nix
+      - name: Lint (statix)
+        run: nix run nixpkgs#statix -- check .
+
+  check:
+    needs: lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@v13
+      - name: nix flake check
+        run: nix flake check --keep-going