feat(domain): add core entities — Club, Member, WorkItem, Shift with state machine

- Create domain entities in WorkClub.Domain/Entities: Club, Member, WorkItem, Shift, ShiftSignup
- Implement enums: SportType, ClubRole, WorkItemStatus
- Add ITenantEntity interface for multi-tenancy support
- Implement state machine validation on WorkItem with C# 14 switch expressions
- Valid transitions: Open→Assigned→InProgress→Review→Done, Review→InProgress (rework)
- All invalid transitions throw InvalidOperationException
- TDD approach: Write tests first, 12/12 passing
- Use required properties with explicit Guid/Guid? for foreign keys
- DateTimeOffset for timestamps (timezone-aware, multi-tenant friendly)
- RowVersion byte[] for optimistic concurrency control
- No navigation properties yet (deferred to EF Core task)
- No domain events or validation attributes (YAGNI for MVP)

.sisyphus/evidence/task-4-state-machine-invalid.txt

@@ -0,0 +1,29 @@
+# Evidence: WorkItem State Machine Invalid Transitions Validation
+
+All invalid transition tests verified to throw InvalidOperationException:
+
+✓ Open_ToDone_Throws - Cannot skip states
+✓ Open_ToInProgress_Throws - Must assign first  
+✓ Assigned_ToDone_Throws - Must go through review
+✓ InProgress_ToOpen_Throws - No backwards transition
+✓ Done_ToAnyStatus_Throws - Terminal state enforcement
+
+State machine implementation correctly enforces:
+- Valid transitions: Open → Assigned → InProgress → Review → Done
+- Rework allowed: Review → InProgress  
+- All invalid transitions throw InvalidOperationException
+
+Implementation in WorkClub.Domain/Entities/WorkItem.cs using C# 14 switch expression:
+```csharp
+public bool CanTransitionTo(WorkItemStatus newStatus) => (Status, newStatus) switch
+{
+    (WorkItemStatus.Open, WorkItemStatus.Assigned) => true,
+    (WorkItemStatus.Assigned, WorkItemStatus.InProgress) => true,
+    (WorkItemStatus.InProgress, WorkItemStatus.Review) => true,
+    (WorkItemStatus.Review, WorkItemStatus.Done) => true,
+    (WorkItemStatus.Review, WorkItemStatus.InProgress) => true,
+    _ => false
+};
+```
+
+All 12 test cases passed successfully.