chore(ci): establish PR governance for agent collaboration (#177)

* chore(ci): establish PR governance for agent collaboration

* docs: add AGENTS playbook and strengthen agent collaboration workflow

---------

Co-authored-by: chumyin <183474434+chumyin@users.noreply.github.com>

.github/workflows/stale.yml

@@ -0,0 +1,44 @@
+name: Stale
+
+on:
+  schedule:
+    - cron: "20 2 * * *"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  stale:
+    permissions:
+      issues: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - name: Mark stale issues and pull requests
+        uses: actions/stale@v9
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          days-before-issue-stale: 21
+          days-before-issue-close: 7
+          days-before-pr-stale: 14
+          days-before-pr-close: 7
+          stale-issue-label: stale
+          stale-pr-label: stale
+          exempt-issue-labels: security,pinned,no-stale,maintainer
+          exempt-pr-labels: no-stale,maintainer
+          remove-stale-when-updated: true
+          exempt-all-assignees: true
+          operations-per-run: 300
+          stale-issue-message: |
+            This issue was automatically marked as stale due to inactivity.
+            Please provide an update, reproduction details, or current status to keep it open.
+          close-issue-message: |
+            Closing this issue due to inactivity.
+            If the problem still exists on the latest `main`, please open a new issue with fresh repro steps.
+          close-issue-reason: not_planned
+          stale-pr-message: |
+            This PR was automatically marked as stale due to inactivity.
+            Please rebase/update and post the latest validation results.
+          close-pr-message: |
+            Closing this PR due to inactivity.
+            Maintainers can reopen once the branch is updated and validation is provided.