feat(ui): compute dependency-chain step/grouping in TasksIslandViewModel

Slice 1 of dependency-chain display: TaskRowViewModel gains DependsOnTaskId
plus the extrinsic ShowAsChainMember/ChainStep/ChainAfterLabel contract for
Slice 2's rail/badge rendering. Regroup's ClassifyItems walks each row's
DependsOnTaskId chain per section, pulls dependents directly under their
head regardless of SortOrder, and falls back to a flat row + label when the
head isn't in the same section (mirrors the ParentInView precedent).
Planning children never join a chain group - parent indent wins - and only
ever carry the label.

No AXAML changes; that's Slice 2.

src/ClaudeDo.Ui/ViewModels/Islands/TaskRowViewModel.cs

@@ -28,12 +28,19 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     [ObservableProperty] private string? _parentTaskId;
     [ObservableProperty] private string? _createdBy;
     [ObservableProperty] private string? _blockedByTaskId;
+    [ObservableProperty] private string? _dependsOnTaskId;
     [ObservableProperty] private bool _isExpanded = true;
     [ObservableProperty] private bool _hasPlanningChildren;
     [ObservableProperty] private bool _hasQueuedSubtasks;
     [ObservableProperty] private bool _showListChip = true;
     [ObservableProperty] private bool _parentFinalized;
     [ObservableProperty] private bool _parentInView = true;
+    // The three below are extrinsic — computed and assigned by TasksIslandViewModel.Regroup
+    // (it alone has the cross-row graph needed to walk DependsOnTaskId chains), mirroring how
+    // ParentInView is assigned by the same pass rather than derived locally.
+    [ObservableProperty] private bool _showAsChainMember;
+    [ObservableProperty] private int? _chainStep;
+    [ObservableProperty] private string? _chainAfterLabel;
     [ObservableProperty] private int _roadblockCount;
     // Only meaningful when Status=Failed; stamped by TaskRunner.MarkFailed. "unknown" for a
     // Failed task that predates this field. Null on every other status.
@@ -370,6 +377,7 @@ public sealed partial class TaskRowViewModel : ViewModelBase
         ParentTaskId      = t.ParentTaskId;
         CreatedBy         = t.CreatedBy;
         BlockedByTaskId   = t.BlockedByTaskId;
+        DependsOnTaskId   = t.DependsOnTaskId;
         RoadblockCount    = t.RoadblockCount;
         FailureReason     = t.FailureReason;
         FailureTurnsUsed  = t.FailureTurnsUsed;