ci: enforce Conventional Commits in PR titles

The squash-merge commit message defaults to the PR title, so
gating titles on Conventional Commits gives release-plz clean
input for changelog generation.

Author: rubenhensen

.github/workflows/pr-title.yml

@@ -0,0 +1,39 @@
+name: PR title
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+      - reopened
+
+permissions:
+  pull-requests: read
+
+jobs:
+  # Enforce Conventional Commits in the PR title so the squash-merge commit
+  # message produces clean changelog entries for release-plz.
+  conventional-commits:
+    name: Conventional Commits
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v6
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            chore
+            docs
+            refactor
+            perf
+            test
+            build
+            ci
+            revert
+          requireScope: false
+          subjectPattern: ^[A-Za-z].+[^.]$
+          subjectPatternError: |
+            The subject "{subject}" must start with a letter and not end with a period.