ci: auto-reopen PRs on issue assignment (#36384)

`require_issue_link.yml` auto-closes external PRs when the author isn't
assigned to the linked issue, but there was no trigger to reopen them
when a maintainer later grants the assignment. Contributors had to
manually edit their PR description to trigger re-evaluation — poor UX
that generated repeat questions. This adds a companion workflow that
listens for issue assignment events and reopens matching PRs
automatically.