ci: add semantic PR title check workflow

Lints PR titles against Conventional Commits via amannn/action-semantic-pull-request,
matching the workflow already in use in postguard-website. Keeps titles consistent
for changelogs and release tooling across the org.

Closes #51

Author: rubenhensen

.github/workflows/pr-title.yml

@@ -0,0 +1,14 @@
+name: PR Title
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+jobs:
+  lint-pr-title:
+    name: Conventional Commit
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}