fix(ci): serialize relay preview retries per PR and drop the listing cap

The nightly sweep ran outside each PR's deploy lock and destroyed a stage
without rechecking the PR, so a reopened PR could lose its new preview. The
listing also stopped at 200 PRs, and a manual dispatch from a PR branch would
have checked that branch out with the cleanup secrets.

Each closed labeled PR now gets a matrix job in the relay-preview-<n> group,
the group deploys use. It confirms the PR is still closed and unlabeled before
destroying, and checks out the default branch explicitly. Listing goes through
the paginated issues API with no cap.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
e06f4f7a81Julius Marminge committed on 10/22/2026, 10:11:17 PMparent554f456

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