refactor(readme_parser): add _href helper to narrow attrGet return type

Extracts a _href(link) helper that returns link.attrGet('href') narrowed
to str (falling back to '') instead of the raw str|int|float|None union.
Replaces all four attrGet('href') or '' call sites with _href(), fixing
ty errors where the widened union leaked into TypedDict url fields.

Co-Authored-By: Claude <noreply@anthropic.com>