ci: respect maintainer reopens on auto-closed PRs (#36115)

When a maintainer manually reopens a PR that was auto-closed by the
`require-issue-link` workflow, skip enforcement so it stays open. Scoped
to PRs carrying the `missing-issue-link` label (i.e. only those closed
by this workflow). Non-org-members reopening their own PRs still go
through normal enforcement.