added formatting CI task

.github/workflows/integrate.yaml

@@ -0,0 +1,33 @@
+on:
+  push:
+    paths:
+      - "**.hs"
+      - "**.nix"
+      - "flake.lock"
+      - "agora.cabal"
+    branches:
+      - master
+  pull_request:
+    paths:
+      - "**.hs"
+      - "**.nix"
+      - "flake.lock"
+      - "agora.cabal"
+jobs:
+  check-formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/cache@v2.1.4
+        name: Cache Stack
+        with:
+          path: ~/.stack
+          key: ${{ runner.os }}-stack-formatting
+          restore-keys: ${{ runner.os }}-stack-
+
+      - run: stack install fourmolu
+        name: Setup
+
+      - run: ./.github/format.sh
+        name: "Run fourmolu"