ci: add commitlint action

To check that commits are according to the Conventional Commits spec.
https://www.conventionalcommits.org/en/v1.0.0/

.github/workflows/commitlint.config.js

@@ -0,0 +1,3 @@
+export default {
+  extends: ["@commitlint/config-conventional"],
+};

.github/workflows/lint-pr.yml

@@ -0,0 +1,20 @@
+name: Lint PR
+
+on:
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  commitlint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - run: >-
+          npx commitlint
+          -g .github/workflows/commitlint.config.js
+          --from ${{ gitea.event.pull_request.base.sha }}
+          --to ${{ gitea.event.pull_request.head.sha }}
+          --verbose