Commit dd424d85f added NameWithOwner to PRRepository for agent-task
listings but didn't update the headRepository GraphQL query to fetch it.
This caused gh pr view --json headRepository to emit an empty
"nameWithOwner":"" field, breaking the pr-create-respects-simple-
pushdefault acceptance test.
Fetch nameWithOwner in the query and update the test assertion to expect
it.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>