CI: manage stale PRs

.github/workflows/stale.yaml

@@ -0,0 +1,28 @@
+name: stale
+on:
+  schedule:
+    - cron: '30 1 * * *'
+  workflow_dispatch:
+
+env:
+  DAYS_BEFORE_PR_CLOSE: 7
+  DAYS_BEFORE_PR_STALE: 30
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v8
+        with:
+          exempt-milestones: true
+
+          # pull request config
+          stale-pr-message: 'This PR has had no activity for ${{ env.DAYS_BEFORE_PR_STALE }} days. It will be automatically closed in ${{ env.DAYS_BEFORE_PR_CLOSE }} days if there is no activity.'
+          close-pr-message: 'This PR has been automatically closed due to inactivity. Feel free to re-open once activity resumes.'
+          delete-branch: ${{ github.event.pull_request.head.repo.full_name == 'commaai/openpilot' }} # only delete branches on the main repo
+          exempt-pr-labels: "ignore stale,needs testing" # if wip or it needs testing from the community, don't mark as stale
+          days-before-pr-stale: ${{ env.DAYS_BEFORE_PR_STALE }}
+          days-before-pr-close: ${{ env.DAYS_BEFORE_PR_CLOSE }}
+
+          # issue config
+          days-before-issue-stale: -1 # ignore issues for now