add merge-schedule workflow

.github/workflows/merge-schedule.yml

@@ -0,0 +1,20 @@
+name: merge-schedule
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - synchronize
+  schedule:
+    # https://crontab.guru/every-hour
+    - cron: 0 * * * *
+jobs:
+  merge_schedule:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: gr2m/merge-schedule-action@v2
+        with:
+          time_zone: Asia/Tokyo
+          require_statuses_success: "true"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}