Fix stale PR message: reference review/urgent label instead of checkbox

[bryanbeverly]

Summary

Updates the stale PR warning message to say "add the review/urgent label" instead of "check the Urgent box in the PR description."

Two reasons:

1. The PR template switched from checkboxes to yes/no fields, so "check the box" is outdated.
2. Not all PRs get the template (API/CLI callers bypass it), so referencing the label directly is accurate regardless of how the PR was created. The exempt-pr-labels: review/urgent check only looks at the label, not the PR body.

Review guidance

• Urgent (needs same-day review): no
• High complexity (non-obvious logic, careful review): no
• Estimated review time: 5 min
• Key files to focus on: .github/workflows/stale-reusable.yml
• Areas you're unsure about: none

Testing

No functional change to workflow logic — only the stale-pr-message string is updated. The exempt-pr-labels and all other actions/stale inputs are unchanged.

Deployment notes

Takes effect on the next scheduled stale run (daily at 01:00 UTC) across all 7 consumer repos. Only affects the message text on newly-staled PRs; PRs that already received the old message are unaffected.

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Note

Low Risk
Low risk: this only changes the text of the actions/stale PR warning message and does not affect workflow behavior or enforcement.

Overview
Updates the stale PR warning message in .github/workflows/stale-reusable.yml to instruct authors to add the review/urgent label (instead of checking an "Urgent" box) to keep a PR from being auto-closed.

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