ci: enforce conventional PR titles (#415)

## Summary
- add PR title workflow using semantic pull request action
- allow conventional commit types used by release-it changelog
generation

## Verification
- parsed workflow YAML locally with Ruby

.github/workflows/pr-title.yml

@@ -0,0 +1,34 @@
+name: PR title
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened, ready_for_review]
+
+jobs:
+  semantic-pr-title:
+    name: Conventional PR title
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: read
+    steps:
+      - name: Validate PR title
+        uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            chore
+            docs
+            style
+            refactor
+            perf
+            test
+            build
+            ci
+            revert
+          requireScope: false
+          subjectPattern: ^(?![A-Z]).+$
+          subjectPatternError: |
+            Subject must not start with uppercase. Use: feat: add checkout, fix(billing): handle retry.