ci: add stale cleanup workflow

.github/workflows/stale-cleanup.yml

@@ -0,0 +1,20 @@
+name: "Stale Cleanup"
+
+on:
+  schedule:
+    # Runs at midnight UTC every Wednesday
+    - cron: "0 0 * * 3"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  stale:
+    uses: braintree/web-sdk-github-actions/.github/workflows/stale-cleanup.yml@cf4b40ff4b46c96aadbfae38806b8de404a98cb3
+    permissions:
+      actions: write
+      issues: write
+      pull-requests: write
+      contents: write
+    with:
+      start-date: "2026-06-01T00:00:00Z"

CONTRIBUTING.md

@@ -13,6 +13,19 @@ Clone this repo, then install the project's dependencies:
 npm install
 ```
 
+## Branch and PR Lifecycle
+
+To keep the repository healthy, an automated workflow runs weekly:
+
+- **Branches**: flagged after 21 days of inactivity; deleted 7 days later if
+  still inactive. To keep a branch, add new commits or remove the bot's comment.
+- **Pull requests**: commented on after 14 days of inactivity; closed 7 days
+  later if still inactive. Closed PRs can be reopened at any time.
+- **Issues**: commented on after 14 days of inactivity; closed 7 days later if
+  still inactive.
+
+You can open a fresh PR or issue at any time if automated cleanup closes yours.
+
 ## Tests
 
 ```