feat(claude-do): depends_on als Menge statt Einzelfeld

Ein Task kann jetzt auf beliebig viele Vorgaenger warten. Die Einzelspalte
tasks.depends_on_task_id wird durch die Beziehungstabelle task_dependencies
ersetzt (Composite-PK, beide FKs ON DELETE CASCADE -- das reproduziert das
alte ON DELETE SET NULL: geloeschter Vorgaenger = Kante weg = Task wieder
pickbar). BlockedByTaskId bleibt unberuehrt.

- Migration AddTaskDependencies kopiert jede bestehende depends_on_task_id
  verlustfrei in die Tabelle, bevor die Spalte fallengelassen wird
- QueuePicker gatet per doppelt negiertem NOT EXISTS: kein Task mit 0 Kanten
  wird blockiert, alle Kanten muessen done sein, eine Kante auf einen
  verschwundenen Task blockiert weiter
- SetDependsOnAsync ersetzt die ganze Menge; Zykluspruefung ist jetzt eine
  BFS ueber den Graphen, faengt also auch A->B->C->A
- blockedReason nennt alle offenen Vorgaenger (External/TaskBlocking, von
  ExternalMcpService und TaskWaitMcpTools gemeinsam genutzt)
- add_task/update_task nehmen dependsOnTaskIds; die alte Skalarform bleibt
  als Ein-Element-Menge inkl. ""-Clear-Sentinel gueltig
- UI: Fan-in-Zeile bleibt flach und listet alle Vorgaenger im After-Label,
  die lineare Kette rendert unveraendert

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

@@ -313,6 +313,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         var entity = await db.Tasks
             .Include(t => t.List)
             .Include(t => t.Worktree)
+            .Include(t => t.Dependencies)
             .FirstOrDefaultAsync(t => t.Id == taskId);
 
         // A newer refresh for this task started while we were reading — its result is fresher.
@@ -453,7 +454,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         {
             await using var db = await _dbFactory.CreateDbContextAsync(ct);
             var filter = _filters.Resolve(list.Id);
-            var baseQuery = db.Tasks.Include(t => t.List).Include(t => t.Worktree);
+            var baseQuery = db.Tasks.Include(t => t.List).Include(t => t.Worktree).Include(t => t.Dependencies);
 
             var filteredList = filter is null
                 ? new List<TaskEntity>()
@@ -722,12 +723,17 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     }
 
     // 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.
+    // holds just the identifying fragment rather than an assembled sentence. Names every
+    // predecessor it can resolve: a task can wait on n of them, and hiding all but the first
+    // would misreport what the row is actually waiting for.
     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;
+        var parts = row.DependsOnTaskIds
+            .Select(id => rowsById.TryGetValue(id, out var p) ? p : null)
+            .Where(p => p is not null)
+            .Select(p => p!.Number > 0 ? $"#{p.Number}" : p.Title)
+            .ToList();
+        return parts.Count == 0 ? null : string.Join(", ", parts);
     }
 
     // Assigns ShowAsChainMember/ChainStep/ChainAfterLabel for every row in `section`, then
@@ -743,7 +749,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
 
         foreach (var r in section)
         {
-            if (string.IsNullOrEmpty(r.DependsOnTaskId))
+            if (r.DependsOnTaskIds.Count == 0)
             {
                 r.ShowAsChainMember = false;
                 r.ChainStep = null;
@@ -751,6 +757,16 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 continue;
             }
 
+            // Several predecessors aren't a chain — the rail draws one line, so a fan-in row
+            // only ever gets the label listing everything it waits on.
+            if (r.DependsOnTaskId is null)
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = BuildAfterLabel(r, rowsById);
+                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)
@@ -1684,6 +1700,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
             entities = await db.Tasks
                 .Include(t => t.List)
                 .Include(t => t.Worktree)
+                .Include(t => t.Dependencies)
                 .Where(t => idSet.Contains(t.Id))
                 .ToListAsync(ct);
             dbOk = true;
@@ -1728,6 +1745,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     // The slice of a row that Regroup reads: which section it lands in, where it sits inside it,
     // and what its chain rail/after-chip says. Only fields UpdateFromEntity actually writes —
     // IsExpanded and HasPlanningChildren are owned elsewhere and would produce false positives.
-    private static (bool, DateTime?, string?, string?, PlanningPhase, int, string) GroupingKey(TaskRowViewModel r) =>
-        (r.Done, r.ScheduledFor, r.ParentTaskId, r.DependsOnTaskId, r.PlanningPhase, r.Number, r.Title);
+    private static (bool, DateTime?, string?, string, PlanningPhase, int, string) GroupingKey(TaskRowViewModel r) =>
+        (r.Done, r.ScheduledFor, r.ParentTaskId, string.Join(",", r.DependsOnTaskIds),
+         r.PlanningPhase, r.Number, r.Title);
 }