Recognize all GitHub issue-link forms in require-issue-link workflow

[jlowin]

The require-issue-link workflow only recognized the Closes #123 hash form when scanning a PR body for its linked issue. GitHub itself also honors Closes owner/repo#123 and the full Closes https://github.com/owner/repo/issues/123 URL — both of which contributors use. A PR linking its issue via either of those forms was wrongly closed as "no link", and assigning the issue afterward never reopened it (the reopen-on-assignment job re-parsed the body with the same regex and skipped the PR). This is exactly what happened to #4319 → #4320.

The shared closing-keyword regex (used identically by both jobs) now matches all three forms GitHub recognizes, scoped to the current repo so a cross-repo reference like Closes otherorg/repo#5 is correctly ignored rather than mis-resolved against our own issue numbering.

// before — only `#123`
/(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\s*:?\s*#(\d+)/gi

// after — `#123`, `owner/repo#123`, and the full issue URL, same-repo only
const repoRef = `${owner}/${repo}`.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
new RegExp(
  '(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\\s*:?\\s*' +
    `(?:${repoRef}#|#|https?://github\\.com/${repoRef}/issues/)(\\d+)`,
  'gi',
);