Shorten stale PR timeline: 30 days to stale, +7 days to close

[Yicong-Huang]

Task Summary

The stale-PR workflow (.github/workflows/stale.yml) currently marks PRs stale after 60 days of inactivity and closes them 30 days later (90 days total). This is too slow; shorten it to 30 days to mark stale and 7 more days to close (37 days total). Escape hatches (pinned label, milestone, draft state, any activity) stay unchanged.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other