Add workflow to mark stale issues and PRs. (#1437)

.github/workflows/stale.yml

@@ -0,0 +1,35 @@
+# This workflow adds a "stale" label to issues and PRs that have had no activity
+# for a specified amount of time, adding a comment to the issue or PR. It does
+# not close the issue or PR.
+#
+# You can adjust the behaviour by modifying this file.
+#
+# For more information, see: https://github.com/actions/stale
+
+name: Mark stale issues and PRs
+
+on:
+  schedule:
+  - cron: '15 5 * * *'
+
+jobs:
+  stale:
+    # Don't run on forks
+    if: github.repository == 'CQCL/tket'
+
+    runs-on: ubuntu-latest
+
+    permissions:
+      pull-requests: write
+      issues: write
+
+    steps:
+    - uses: actions/stale@v9
+      with:
+        days-before-issue-stale: 120
+        days-before-pr-stale: 30
+        days-before-close: -1
+        stale-issue-label: 'stale'
+        stale-pr-label: 'stale'
+        stale-issue-message: 'This issue has been automatically marked as stale.'
+        stale-pr-message: 'This pull request has been automatically marked as stale.'