ci: add workflow to close stale pull requests

Marks pull requests as stale after 60 days of inactivity and closes them
14 days later. Issues are excluded. Runs daily and can be triggered
manually via workflow_dispatch.

.github/workflows/stale.yml

@@ -0,0 +1,27 @@
+name: Close Stale Pull Requests
+permissions: read-all
+
+on:
+  schedule:
+    - cron: '0 2 * * *'
+  workflow_dispatch:
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - uses: actions/stale@b5d41d4e1d5dceea10e7104786b73624c18a190f # v10.2.0
+        with:
+          days-before-pr-stale: 60
+          days-before-pr-close: 14
+          stale-pr-label: stale
+          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 14 days.
+          close-pr-message: >
+            This pull request has been automatically closed due to inactivity.
+            Please feel free to reopen it if you would like to continue working on it.
+          days-before-issue-stale: -1
+          days-before-issue-close: -1