Clean up dataset module exports

[iliall]

Summary

IssueDataset was previously exported from src/dataset/__init__.py, but is now an abstract base class after the refactor to support local JSON files. This could confuse users who try to instantiate it directly.

Current State

from .loader import IssueDataset, load_issues

__all__ = ["IssueDataset", "load_issues"]

Suggested Change

Only export load_issues as the public API:

from .loader import load_issues

__all__ = ["load_issues"]

Priority

Low: internal project; no external consumers affected.