When the `require_issue_link` workflow closes a PR and posts an
enforcement comment, that comment was never cleaned up after the
situation resolved — leaving a stale "automatically closed" message
visible on reopened PRs. Now all three resolution paths (maintainer
bypass, author fixing the issue link, and contributor assignment)
minimize the enforcement comment as outdated via GraphQL. The cleanup is
best-effort: failures log a warning but never block the primary workflow
logic (label removal, bypass, reopen).
`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.
