feat(planning): let CreateChildTask set maxTurns on child tasks

Planning sessions could already steer a subtask's model but not its turn
budget, so a visibly large subtask would still die at the global default
turn limit. maxTurns is optional (default null = inherit list/global
default, matching model); 0/negative values are rejected as invalid input,
consistent with the existing model-alias validation.

tests/ClaudeDo.Worker.Tests/Repositories/TaskRepositoryPlanningTests.cs

@@ -119,6 +119,47 @@ public sealed class TaskRepositoryPlanningTests : IDisposable
             _tasks.CreateChildAsync("nonexistent-parent-id", "t", null, null));
     }
 
+    [Fact]
+    public async Task CreateChildAsync_NoMaxTurns_ChildInherits()
+    {
+        var listId = await CreateListAsync();
+        var parent = MakeTask(listId, phase: PlanningPhase.Active);
+        await _tasks.AddAsync(parent);
+
+        var child = await _tasks.CreateChildAsync(parent.Id, "child", null, null);
+
+        Assert.Null(child.MaxTurns);
+        var loaded = await _tasks.GetByIdAsync(child.Id);
+        Assert.Null(loaded!.MaxTurns);
+    }
+
+    [Fact]
+    public async Task CreateChildAsync_WithMaxTurns_Persists()
+    {
+        var listId = await CreateListAsync();
+        var parent = MakeTask(listId, phase: PlanningPhase.Active);
+        await _tasks.AddAsync(parent);
+
+        var child = await _tasks.CreateChildAsync(parent.Id, "child", null, null, maxTurns: 200);
+
+        Assert.Equal(200, child.MaxTurns);
+        var loaded = await _tasks.GetByIdAsync(child.Id);
+        Assert.Equal(200, loaded!.MaxTurns);
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-5)]
+    public async Task CreateChildAsync_InvalidMaxTurns_Throws(int maxTurns)
+    {
+        var listId = await CreateListAsync();
+        var parent = MakeTask(listId, phase: PlanningPhase.Active);
+        await _tasks.AddAsync(parent);
+
+        await Assert.ThrowsAsync<ArgumentException>(() =>
+            _tasks.CreateChildAsync(parent.Id, "child", null, null, maxTurns: maxTurns));
+    }
+
     [Fact]
     public async Task SetPlanningStartedAsync_IdleTask_TransitionsToActivePhase()
     {