refactor(fetch_github_stars): replace manual slice loop with itertools.batched

Use itertools.batched (stdlib since Python 3.12, targeted by this project)
instead of manual range(0, N, BATCH_SIZE) slicing. Loosen fetch_batch,
build_graphql_query, and parse_graphql_response signatures from list[str]
to Sequence[str] since batched yields tuples.

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