chore: add stale PR workflow

Adds .github/workflows/close-stale-prs.yaml to mark PRs as stale after
7 days of inactivity and close them 7 days later. Draft PRs are exempt.
This keeps the PR queue clean as AI agent throughput increases.

Ticket: AI-595
Session-Id: b8d5e9df-80dc-4cbe-91f0-590831e14bb9
Task-Id: 3cbad070-b9c6-4af0-847d-e62d87686533

Author: ericcrosson-bitgo

.github/actionlint.yaml

@@ -0,0 +1,3 @@
+self-hosted-runner:
+  labels:
+    - ubuntu-slim

.github/workflows/close-stale-prs.yaml

@@ -0,0 +1,30 @@
+name: Close stale PRs
+
+on:
+  schedule:
+    - cron: '0 1 * * *'
+  workflow_dispatch:
+
+jobs:
+  stale:
+    runs-on: ubuntu-slim
+    permissions:
+      issues: write
+      pull-requests: write
+    steps:
+      - uses: actions/stale@v10
+        with:
+          days-before-issue-stale: -1
+          days-before-issue-close: -1
+          days-before-pr-stale: 7
+          days-before-pr-close: 7
+          stale-pr-label: stale
+          stale-pr-message: >
+            This PR has been marked as stale because it has had no activity for 7 days.
+            Remove the stale label or comment to keep it open. It will be closed
+            automatically in 7 more days if no activity occurs.
+          close-pr-message: >
+            This PR was closed automatically because it has been stale for 14 days with
+            no activity.
+          exempt-draft-pr: true
+          operations-per-run: 30