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

tests/ClaudeDo.Worker.Tests/Queue/QueuePickerTests.cs

@@ -50,7 +50,7 @@ public sealed class QueuePickerTests : IDisposable
         bool taskAgentTag = false,
         int? sortOrder = null,
         bool isManual = false,
-        string? dependsOn = null)
+        params string[] dependsOn)
     {
         var task = new TaskEntity
         {
@@ -61,11 +61,12 @@ public sealed class QueuePickerTests : IDisposable
             CreatedAt = createdAt ?? DateTime.UtcNow,
             ScheduledFor = scheduledFor,
             BlockedByTaskId = blockedBy,
-            DependsOnTaskId = dependsOn,
             CommitType = "feat",
             IsManual = isManual,
         };
         await _tasks.AddAsync(task);
+        if (dependsOn.Length > 0)
+            await _ctx.SeedDependsOnAsync(task.Id, dependsOn);
         if (sortOrder is not null)
         {
             task.SortOrder = sortOrder.Value;
@@ -117,7 +118,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Idle);
-        await SeedAsync(listId, dependsOn: dependency.Id);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
 
         // The dependency is only Idle (not Done), so both stay unclaimed: the dependent is
         // gated on depends_on_task_id, and the dependency itself was never queued.
@@ -129,7 +130,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Done);
-        var dependent = await SeedAsync(listId, dependsOn: dependency.Id);
+        var dependent = await SeedAsync(listId, dependsOn: [dependency.Id]);
 
         var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
 
@@ -142,7 +143,7 @@ public sealed class QueuePickerTests : IDisposable
     {
         var listId = await CreateListAsync();
         var dependency = await SeedAsync(listId, status: TaskStatus.Failed);
-        await SeedAsync(listId, dependsOn: dependency.Id);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
         var independent = await SeedAsync(listId, createdAt: DateTime.UtcNow.AddMinutes(1));
 
         // A Failed dependency does not cascade-cancel or auto-unblock the dependent (unlike the
@@ -155,6 +156,55 @@ public sealed class QueuePickerTests : IDisposable
         Assert.Null(second);
     }
 
+    [Fact]
+    public async Task ClaimNextAsync_Skips_TaskWhoseDependsOnCancelled()
+    {
+        var listId = await CreateListAsync();
+        var dependency = await SeedAsync(listId, status: TaskStatus.Cancelled);
+        await SeedAsync(listId, dependsOn: [dependency.Id]);
+
+        Assert.Null(await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Skips_TaskWithOneOfTwoDependenciesUnmet()
+    {
+        var listId = await CreateListAsync();
+        var done = await SeedAsync(listId, status: TaskStatus.Done);
+        var running = await SeedAsync(listId, status: TaskStatus.Running);
+        await SeedAsync(listId, dependsOn: [done.Id, running.Id]);
+
+        Assert.Null(await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Claims_TaskOnceAllDependenciesAreDone()
+    {
+        var listId = await CreateListAsync();
+        var first = await SeedAsync(listId, status: TaskStatus.Done);
+        var second = await SeedAsync(listId, status: TaskStatus.Done);
+        var dependent = await SeedAsync(listId, dependsOn: [first.Id, second.Id]);
+
+        var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
+
+        Assert.NotNull(picked);
+        Assert.Equal(dependent.Id, picked!.Id);
+    }
+
+    [Fact]
+    public async Task ClaimNextAsync_Claims_TaskWithoutAnyDependency()
+    {
+        // The gating clause is "no edge whose predecessor isn't done" — a task with zero edges
+        // must stay eligible, which the doubly-negated form is easy to get wrong.
+        var listId = await CreateListAsync();
+        var task = await SeedAsync(listId);
+
+        var picked = await _picker.ClaimNextAsync(DateTime.UtcNow, CancellationToken.None);
+
+        Assert.NotNull(picked);
+        Assert.Equal(task.Id, picked!.Id);
+    }
+
     [Fact]
     public async Task ClaimNextAsync_Picks_TasksWithoutAgentTag()
     {