Support cross-repo issue links in check-pr-issue-link

[glageju]

Closes #5

Summary

• Extends check-pr-issue-link to recognize cross-repo closing references (e.g. Closes org/repo followed by the number) alongside the existing same-repo form.
• Each reference is validated against its own owner/repo via the GitHub API; same-repo behavior is unchanged.
• Per-reference rendering in the warning/heads-up comments: same-repo refs use the bare number form, cross-repo refs include the owner/repo prefix.
• Unverifiable cross-repo references (typo, private repo, or repo the workflow's GITHUB_TOKEN cannot see) are treated the same as same-repo 404s — reported as not found.

Notable changes

• LINK_REGEX broadened. The alternation (?:owner/repo#|#?) keeps the hash optional for same-repo refs and required for cross-repo refs in a single pattern.
• Matches are normalized into { owner, repo, number, isCrossRepo } references, deduped by the full owner/repo/number tuple.
• README updated with a Supported reference forms section and a cross-repo example.

Test plan

Verified end-to-end in a sandbox caller repo pointing uses: at this branch. All five states produced the expected comment/label:

• Empty body → Missing Issue Link comment + needs-issue-link label.
• Same-repo valid reference → comment and label cleared.
• Cross-repo reference to a nonexistent issue → Invalid Issue Reference comment in the new cross-repo wording; label applied.
• Mixed (valid same-repo + invalid cross-repo) → Heads-up comment listing only the cross-repo invalid; label removed.
• Cross-repo valid reference → comment and label cleared.

Note: the PR description intentionally avoids bare issue tokens in prose — the workflow scans all text (including inline code), so any #N or owner/repo#N would be treated as a real reference.