Add GH actions workflow to replace mergeable bot

Author: yut23

.github/mergeable.yml

@@ -0,0 +1,37 @@
+# based on https://stackoverflow.com/a/75036059 and
+# https://github.com/astropy/astropy/blob/main/.github/workflows/check_milestone.yml
+name: Check PR title and labels
+
+on:
+  # So it cannot be skipped.
+  pull_request_target:
+    types: [opened, edited, synchronize, labeled, unlabeled]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+jobs:
+  check-title:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          script: |
+            if (/^\[?WIP\b/i.test(context.payload.pull_request.title)) {
+              core.setFailed("WIP pull requests can't be merged.");
+            }
+
+  check-labels:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          script: |
+            const requiredLabels = ['bug', 'enhancement', 'new feature', 'docs', 'infrastructure', 'dead code', 'refactor'];
+            let labels = context.payload.pull_request.labels;
+
+            core.info(`Current labels: ${labels.map(l => l.name).join(', ')}`);
+            if (labels.filter(l => requiredLabels.includes(l.name)).length === 0) {
+              core.setFailed('Please label this pull request with one of: bug, enhancement, new feature, docs or infrastructure.');
+            }