ci: add stale PR workflow

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

.github/workflows/stale-pr.yml

@@ -0,0 +1,19 @@
+name: 'Close stale pull requests'
+
+on:
+  schedule:
+  - cron: '0 0 * * *'
+
+permissions:
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-24.04
+    steps:
+    - uses: actions/stale@v9
+      with:
+        stale-pr-message: 'This pull request has been marked as stale because it has been open (more than) 30 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this pull request will automatically be closed in 7 days. Note, that you can always re-open a closed pull request at any time.'
+        days-before-pr-stale: 30
+        days-before-pr-close: 7
+        stale-pr-label: 'stale'