v0.6.1

Changes

Add WorkItem.get_parent_id() to retrieve the numeric ID of a work
item's parent from already-fetched relation data (no extra API call).

Add PullRequest.add_work_item_ref() as an idempotent convenience
wrapper around iter_work_item_ids + set_work_item_refs: issues one
GET and one PATCH when the item is new, GET-only when already linked.