ci(github-actions): lint PR title instead of commits

.github/workflows/commitlint.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     types:
       - opened
+      - edited
       - reopened
       - synchronize
       - ready_for_review
@@ -29,5 +30,7 @@ jobs:
       - name: Install npm packages
         run: npm ci
 
-      - name: Validate PR commits with commitlint
-        run: npx commitlint --from ${{ github.event.pull_request.base.sha }} --verbose
+      - name: Lint PR title
+        env:
+          PR_TITLE: ${{ github.event.pull_request.title }}
+        run: echo "$PR_TITLE" | npx commitlint --verbose

CONTRIBUTING.md

@@ -125,7 +125,10 @@ Creating a release means:
 ### Commit Messages
 
 The changelog and the version number are determined by the commit messages.
-Therefore, commit messages should follow the [Conventional Commits](https://www.conventionalcommits.org/) specification.
+Therefore, commit messages on the `main` branch should follow the [Conventional Commits](https://www.conventionalcommits.org/) specification.
+Since we squash-merge pull requests, the PR title should also follow conventional commits
+(because it will become the commit message of the squashed commit).
+
 Also refer to the Release Please documentation for more information on how to write commit messages.
 If you want to indicate a breaking change, you can use the `BREAKING CHANGE` keyword in the commit message,
 followed by the description of the breaking change.