ci: delete the staging branch when a preview pull request closes

[tisonkun]

Summary

The deploy workflow publishes every preview/<name> build to a generated site/<name>-staging branch, and nothing removed those branches afterwards — three stale staging branches had accumulated, including one from a pull request that was closed unmerged.

This adds a cleanup-staging workflow triggered on pull_request: [closed]: when a same-repo pull request from a preview/<name> branch closes (merged or not), it deletes the matching site/<name>-staging branch. AGENTS.md documents the behavior and the manual cleanup option.

The three existing stale branches (site/docusaurus-staging, site/homepage-staging, site/readme-homepage-staging) have already been deleted as a one-time cleanup.

Design Notes

• Deleting a staging branch does not unpublish its staged site: the deleted previews kept serving HTTP 200 for 30+ minutes after removal. The workflow therefore only guarantees that deployment output does not accumulate in the repository; staged.apache.org content is pruned by ASF infrastructure on its own schedule (an infra ticket may be needed if stale previews turn out to persist indefinitely).

Staging preview: https://asyncband-cleanup-staging.staged.apache.org/