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;

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

@@ -580,9 +580,124 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 open.Add(r);
         }
 
+        // Dependency chains are resolved and pulled together per section (not on the
+        // pre-split `flat` list): a chain head might land in a different section than its
+        // dependent (e.g. a Done head in Completed, an open dependent in Open) — in that case
+        // the two are never rendered adjacent, so the dependent must fall back to a flat row
+        // with a label rather than an orphaned rail. Whether the head is resolvable at all
+        // still uses the whole-Items graph (rowsById), independent of section.
+        var rowsById = Items.ToDictionary(r => r.Id);
+        overdue   = ApplyChainGrouping(overdue, rowsById);
+        open      = ApplyChainGrouping(open, rowsById);
+        completed = ApplyChainGrouping(completed, rowsById);
+
         return (overdue, open, completed);
     }
 
+    // Cycles are rejected by TaskStateService.SetDependsOnAsync, so this should always
+    // terminate quickly — capped anyway against a corrupt/legacy row forming a loop.
+    private const int MaxChainWalkDepth = 64;
+
+    private readonly record struct ChainWalkResult(TaskRowViewModel Head, int Step);
+
+    // Walks DependsOnTaskId back to its root. Returns null when the chain can't be resolved —
+    // either the row has no predecessor at all, or a predecessor along the way isn't loaded
+    // into the current Items (e.g. filtered out of this list/view).
+    private static ChainWalkResult? WalkChainHead(TaskRowViewModel row, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        var current = row;
+        var step = 0;
+        var visited = new HashSet<string> { row.Id };
+        while (!string.IsNullOrEmpty(current.DependsOnTaskId))
+        {
+            if (!rowsById.TryGetValue(current.DependsOnTaskId, out var predecessor)) return null;
+            if (step >= MaxChainWalkDepth || !visited.Add(predecessor.Id)) return null;
+            current = predecessor;
+            step++;
+        }
+        return step == 0 ? null : new ChainWalkResult(current, step);
+    }
+
+    // Content only — "after " + localization is Slice 2's concern (the view), which is why this
+    // holds just the identifying fragment rather than an assembled sentence.
+    private static string? BuildAfterLabel(TaskRowViewModel row, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        if (string.IsNullOrEmpty(row.DependsOnTaskId)) return null;
+        if (!rowsById.TryGetValue(row.DependsOnTaskId, out var predecessor)) return null;
+        return predecessor.Number > 0 ? $"#{predecessor.Number}" : predecessor.Title;
+    }
+
+    // Assigns ShowAsChainMember/ChainStep/ChainAfterLabel for every row in `section`, then
+    // returns a re-ordered copy with chain dependents pulled directly under their head
+    // (ascending ChainStep), regardless of their SortOrder-derived position — mirroring how
+    // planning children already sit right after their parent regardless of Items order.
+    private static List<TaskRowViewModel> ApplyChainGrouping(
+        List<TaskRowViewModel> section, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        var sectionIds = section.Select(r => r.Id).ToHashSet();
+        var headIdOf = new Dictionary<TaskRowViewModel, string>();
+        var stepOf = new Dictionary<TaskRowViewModel, int>();
+
+        foreach (var r in section)
+        {
+            if (string.IsNullOrEmpty(r.DependsOnTaskId))
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = null;
+                continue;
+            }
+
+            // A planning child keeps its parent indent — chain membership only ever shows as a
+            // label for it, never as a second, nested rail (design: "parent wins").
+            if (r.IsChild)
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = BuildAfterLabel(r, rowsById);
+                continue;
+            }
+
+            var walk = WalkChainHead(r, rowsById);
+            if (walk is { } w && sectionIds.Contains(w.Head.Id))
+            {
+                r.ShowAsChainMember = true;
+                r.ChainStep = w.Step;
+                r.ChainAfterLabel = null;
+                headIdOf[r] = w.Head.Id;
+                stepOf[r] = w.Step;
+            }
+            else
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = BuildAfterLabel(r, rowsById);
+            }
+        }
+
+        if (headIdOf.Count == 0) return section;
+
+        var membersByHeadId = section
+            .Where(headIdOf.ContainsKey)
+            .GroupBy(r => headIdOf[r])
+            .ToDictionary(g => g.Key, g => g.OrderBy(r => stepOf[r]).ToList());
+
+        var ordered = new List<TaskRowViewModel>(section.Count);
+        var consumed = new HashSet<string>();
+        foreach (var r in section)
+        {
+            if (consumed.Contains(r.Id)) continue;
+            if (headIdOf.ContainsKey(r)) continue; // placed via its head below, in step order
+            ordered.Add(r);
+            consumed.Add(r.Id);
+            if (membersByHeadId.TryGetValue(r.Id, out var members))
+                foreach (var m in members)
+                    if (consumed.Add(m.Id))
+                        ordered.Add(m);
+        }
+        return ordered;
+    }
+
     private void UpdateSubtitle()
     {
         var now     = DateTime.Now;