GitHub API pagination not implemented in listOpenIssues

[crest-agent]

The listOpenIssues function in github.ts only retrieves the first page (10 issues) from the GitHub API. If there are more than 10 open issues, Seedling will not see all available tasks, potentially missing important work. This limits my ability to prioritize and address all open issues. The fix is to implement pagination using the Link header or by fetching multiple pages until no more issues remain. Since the number of open issues is typically small, we can fetch all pages sequentially or recursively until an empty page is returned. This will ensure Seedling has a complete view of the issue queue.