feat(workitems): Add more fields to WorkItem

Adds the following fields to the WorkItem struct, sourced from the corresponding GraphQL widgets:

• Color
• Confidential
• DueDate / StartDate
• HealthStatus
• IterationID
• Labels
• LinkedItems
• MilestoneID
• ParentID
• Status
• Weight

The workItemFeaturesGQL struct is refactored from a flat, ad-hoc structure into individual typed widget structs (one per widget), each with its own nil-safe unwrap() method. This matches the shape of the GraphQL API, where each widget is an independent nullable object, and makes it straightforward to add further widgets in the future.

The new WorkItemIID type is used to reference other work items in the ParentID and LinkedItems fields. When introducing the work items API, we made a conscious decision to identify work items by namespace path and IID rather than their global ID. The new type combines these two pieces of identifying information so it is easier to return and work with.

Two smaller fixes are included:

• userCoreBasicGQL.unwrap() now has a pointer receiver and handles nil receivers gracefully, preventing a panic when the author field is absent on a work item.

Issue: Feature Request: Add support for Work Items (#2213)