ci: add PR title validation for conventional commits

Enforce conventional commit format on PR titles using
action-semantic-pull-request. Combined with squash merge,
this ensures release-please can parse commit history correctly.

Author: konokenj

.github/workflows/commitlint.yml

@@ -0,0 +1,16 @@
+name: PR Title
+
+on:
+  pull_request_target:
+    types: [opened, edited, synchronize]
+
+permissions:
+  pull-requests: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v6
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}