ci: validate PR title instead of commit messages (#2)

## Summary

The original `commitlint` workflow used
`wagoid/commitlint-github-action@v6` which validates commit messages,
not PR titles. Because the repo uses squash-merge with PR title as the
commit message, the PR title is the right surface to validate.

Replace with `amannn/action-semantic-pull-request@v5` which validates
the PR title directly. Job name kept as `commitlint` so branch
protection required-check context still resolves.

## Test plan

- Verified the current action lets bad PR titles through if commits
inside are conventional (closed test PR #1)
- This PR title follows the policy; new action should pass
- Post-merge smoke test: open a PR with a bad title like `test change`;
expect fail

Co-authored-by: lxcong <lxhtheresa@gmail.com>

.github/workflows/commitlint.yml

@@ -11,10 +11,23 @@ jobs:
   commitlint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: wagoid/commitlint-github-action@v6
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
-          configFile: .commitlintrc.json
-          firstParent: false
-          failOnWarnings: false
-          helpURL: https://www.conventionalcommits.org/
+          types: |
+            feat
+            fix
+            docs
+            chore
+            refactor
+            test
+            ci
+            perf
+            style
+          requireScope: false
+          subjectPattern: '^[a-z].+[^.]$'
+          subjectPatternError: |
+            The subject "{subject}" found in the PR title "{title}"
+            did not match the pattern. The subject must start with a lowercase
+            character and not end with a period.