* ci: require an open issue for first-time contributor PRs
Gate anyone who is not a returning contributor, and allow the PR only when a closing keyword points at an open issue in this repo.
* ci: accept any open issue mention for first-timer PRs
Drop the closing-keyword regex so #123, owner/repo#N, or an issue URL is enough when that issue is open.
* ci: ignore foreign owner/repo#N in first-timer issue gate
Bare #123 no longer matches the suffix of other/repo#123, so an open local issue cannot keep that PR open.
