style: check formatting using nixpkgs standard and

fail early in CI to enforce it

.forgejo/workflows/check.yml

@@ -46,6 +46,10 @@ jobs:
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
           useDaemon: false
 
+      - name: Check formatting
+        run: |
+          nix --accept-flake-config --access-tokens '' develop --command treefmt --fail-on-change
+
       - name: Run flake checks
         run: |
           # Prevent cache garbage collection by creating GC roots

treefmt.toml

@@ -0,0 +1,24 @@
+[formatter.nix]
+command = "nix"
+options = ["fmt"]
+includes = ["*.nix"]
+excludes = []
+
+[formatter.prettier]
+command = "prettier"
+options = ["--write"]
+includes = [
+  "*.json",
+  "*.yaml",
+  "*.md",
+]
+
+[formatter.shell]
+command = "shfmt"
+options = [
+  "-s",
+  "-w",
+  "-i",
+  "2",
+]
+includes = ["*.sh"]