ci: add workflow to close PRs inactive for 2+ weeks

.github/workflows/stale.yml

@@ -0,0 +1,26 @@
+name: Close stale PRs
+
+on:
+  schedule:
+    - cron: "0 0 * * *"
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          days-before-pr-stale: 7
+          days-before-pr-close: 7
+          stale-pr-message: "This PR has been inactive for 7 days and will be closed in 7 days if there is no activity."
+          close-pr-message: "Closing this PR due to inactivity. Feel free to reopen if still needed."
+          stale-pr-label: stale
+          exempt-pr-labels: do-not-close
+          operations-per-run: 1000
+          days-before-issue-stale: -1
+          days-before-issue-close: -1