Add a stale CI workflow (stellar#1775)

.github/workflows/stale.yml

@@ -0,0 +1,23 @@
+name: 'Stale Issues / PRs'
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 18 * * *' # approx 9:30am daily
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          debug-only: false
+          days-before-stale: 30
+          days-before-close: 30
+          stale-issue-message: 'This issue is stale because it has been assigned for 30 days with no activity. It will be closed in 30 days unless the stale label is removed, and the assignee is removed or updated.'
+          stale-pr-message: 'This pull request is stale because it has been open for 30 days with no activity. It will be closed in 30 days unless the stale label is removed.'
+          stale-issue-label: stale
+          stale-pr-label: stale
+          remove-stale-when-updated: true
+          delete-branch: true
+          include-only-assigned: true