Add GH action to close stale PRs

.github/workflows/stale.yml

@@ -0,0 +1,27 @@
+# This workflow warns and then closes PRs that have had no activity for 90 days.
+#
+# For more information, see:
+# https://github.com/actions/stale
+name: Close stale pull requests
+
+on:
+  schedule:
+  - cron: '0 18 * * *' # Run the workflow every day at 6PM UTC (10AM PST).
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+
+    steps:
+    - uses: actions/stale@v5
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-pr-message: 'This pull request has been automatically marked as stale because it has been inactive for 90 days. Remove stale label or comment or this PR will be closed in 7 days.'
+        stale-pr-label: 'stale'
+        days-before-pr-stale: 90 # 3 months
+        days-before-pr-close: 7
+        days-before-issue-stale: -1
+        days-before-issue-close: -1

CONTRIBUTING.md

@@ -21,3 +21,7 @@ $ git commit -s -m 'This is my commit message'
 ```
 
 Visual Studio Code has a setting, `git.alwaysSignOff` to automatically add a Signed-off-by line to commit messages. Search for "sign-off" in VS Code settings to find it and enable it.
+
+## Inactive Pull Requests
+
+Pull requests that have been inactive for 90 days will be marked with a stale label. They will automatically be closed after a subsequent 7 days of inactivity. This timeframe may be adjusted in the future based on project needs.