Enforce repository formatting in GitHub CI (#2023)

Summary:
Pull Request resolved: #2023

Add a dedicated GitHub Actions check for the repository-owned formatter contract and document the same commands for contributors. The job provisions the Java and Python runtimes needed by Yoga’s Kotlin and Python formatters before running `yarn format-check`.

___

Reviewed By: cipolleschi

Differential Revision: D119662616

fbshipit-source-id: eeb03aa263d3e528ceb5b7f0884ce232153608ee

Author: cortinico

.github/workflows/validate-format.yml

@@ -0,0 +1,36 @@
+name: Validate Formatting
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+      - 'release-*'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  format:
+    name: Format
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Setup JavaScript
+        uses: ./.github/actions/setup-js
+
+      - name: Setup Java
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Check formatting
+        run: yarn format-check

CONTRIBUTING.md

@@ -14,10 +14,14 @@ We actively welcome your pull requests.
 1. Fork the repo and create your branch from `main`.
 2. If you've added code that should be tested, add tests
 3. If you've changed APIs, update the documentation.
-4. Ensure the test suite passes.
-5. Make sure your code lints.
+4. Format the repository with `yarn format`.
+5. Ensure the test suite passes and `yarn lint` succeeds.
 6. If you haven't already, complete the Contributor License Agreement ("CLA").
 
+`yarn format-check` checks all maintained JavaScript and documentation, C-family,
+Kotlin, and Python sources without modifying them. Running all formatters requires
+Node and Yarn, Java 17 or newer, and Python 3 with pip.
+
 ## Contributor License Agreement ("CLA")
 
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