Add workflow to mark stale issues and PRs

- Runs daily at 22:30 UTC+08
- Marks issues stale after 90 days
- Closes stale issues after 7 more days

.github/workflows/stale.yaml

@@ -0,0 +1,27 @@
+# .github/workflows/stale.yml
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+    - cron: '30 22 * * *' # run at 22:30+08 every day
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          days-before-stale: 90 # 90 days
+          days-before-close: 7 # 7 days after marked as stale
+          stale-issue-message: 'This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.'
+          close-issue-message: 'This issue has been automatically closed because it has not had recent activity. Please open a new issue if you still have this problem.'
+          stale-pr-message: 'This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.'
+          close-pr-message: 'This pull request has been automatically closed because it has not had recent activity.'
+          # If there are specific labels, exempt them from being marked as stale, for example:
+          exempt-issue-labels: 'enhancement,tracked'
+          # exempt-pr-labels: 'bug,enhancement,help wanted'
+          repo-token: ${{ secrets.GITHUB_TOKEN }} # token provided by GitHub