feat: add stale pending-contributor → closing-soon workflow

[chaodu-agent]

Summary

Adds a workflow that automatically applies closing-soon to open PRs where pending-contributor has been applied for more than 2 days without action.

Behavior

• Runs hourly + manual workflow_dispatch
• Scans open PRs matching: is:pr is:open label:pending-contributor -label:closing-soon
• Checks when pending-contributor was last applied via issue events timeline
• If ≥ 2 days since label was applied → adds closing-soon

Lifecycle

This completes the PR label lifecycle:

1. feat: add hourly needs-rebase label workflow #450 needs-rebase — hourly conflict detection → adds needs-rebase + pending-contributor
2. feat: add pending-maintainer label workflow #451 pending-maintainer — flips to pending-maintainer when all conditions met
3. This PR — escalates stale pending-contributor → closing-soon after 2 days
4. Existing close-stale-prs.yml — auto-closes closing-soon PRs after 3 more days

Files Changed

• .github/workflows/stale-contributor.yml (new)

[github-actions]

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