fix(mirror): filter cross-repo closing refs

[Desel72]

Summary

Filter GitHub closing issue references to the PR's own repository before storing them in pull_requests.closing_issue_numbers.

• Fetch repository { nameWithOwner } for each closingIssuesReferences node.
• Keep only issue numbers whose closing-reference repository matches the PR repository case-insensitively.
• Prevent cross-repo close references like Closes ownerB/repoB#123 from being reinterpreted as ownerA/repoA#123 by pr_linked_issues or solved_by_pr reconciliation.

Related Issues

Closes #35

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• npm run build
• npm run lint
• npm run format:check
• git diff --check

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added