ci: Enforce conventional commit format in PR title (#398)

Closes #66

.github/workflows/lint-pr-title.yml

@@ -0,0 +1,35 @@
+name: Lint PR title
+
+on:
+  pull_request:
+    types: [opened, reopened, edited]
+
+permissions:
+  pull-requests: read
+
+jobs:
+  lint-pr-title:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+      - name: Install commitlint
+        run: |
+          npm install @commitlint/config-conventional@19.5.0
+          npm install commitlint@19.2.0
+      - name: Generate config file
+        # Here we define our custom rules for commitlint
+        # Reference of all rules: https://commitlint.js.org/reference/rules.html
+        run: |
+          tee -a .commitlintrc.yml << END
+          extends: ["@commitlint/config-conventional"]
+          rules:
+            "subject-case": [2, "always", ["sentence-case"]]
+          END
+      - name: Lint PR title
+        # Use an environment variable to avoid a security vulnerability
+        # https://stackoverflow.com/a/76679447
+        env:
+          PR_TITLE: ${{ github.event.pull_request.title }}
+        run: echo $PR_TITLE | npx commitlint --verbose

CONTRIBUTING.md

@@ -70,6 +70,14 @@ Then, you can access the local build via:
 open doc/_build/html/index.html
 ```
 
+### PR format
+
+We use the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/#summary) format, and we automatically check that the PR title fits this format.
+In particular, commits are "sentence case", meaning "fix: Fix issue" passes, while "fix: fix issue" doesn't.
+Our custom set of rules is in [commitlint.config.js](./commitlint.config.js).
+
+Generally the description of a commit should start with a verb in the imperative voice, so that it would properly complete the sentence: "When applied, this commit will [...]".
+
 ## Help for common issues
 
 ### `make build-frontend` doesn't work!