ci: add workflow for managing stale PRs & issues

.github/workflows/stale.yml

@@ -0,0 +1,26 @@
+name: Mark / close stale issues and pull requests
+
+on:
+  schedule:
+    - cron: '0 0 * * *' # Runs every day at midnight UTC
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          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 within 15 days.'
+          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 within 15 days.'
+          days-before-stale: 30
+          days-before-close: 45
+          stale-issue-label: 'stale'
+          stale-pr-label: 'stale'
+          exempt-issue-labels: 'evergreen'
+          exempt-pr-labels: 'evergreen'