feat(worker): dependsOn via MCP + honest staleness signal in merge preview

Adds a user/MCP-declared task dependency (DependsOnTaskId) distinct from the
planning chain's internal BlockedByTaskId: add_task/update_task can set it,
the queue picker skips a Queued task until the dependency reaches Done, a
Failed/Cancelled dependency leaves the dependent blocked instead of starving
silently, and setting a link rejects self-reference/unknown-id/cycles.
get_task/list_tasks/batch_get_tasks now report blocked/blockedReason, and
wait_for_task_change reports "Blocked" immediately instead of running out its
timeout on a task the picker will never claim.

preview_merge/preview_merge_set gain staleFiles: files a branch touches that
the target branch also changed since the branch's fork point, a more honest
staleness signal than `behind` alone.

tests/ClaudeDo.Worker.Tests/State/TaskStateServiceTests.cs

@@ -472,6 +472,100 @@ public sealed class TaskStateServiceTests : IDisposable
         Assert.True(_built.WakeCount() > wakesBefore);
     }
 
+    // ─── SetDependsOnAsync ────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SetDependsOnAsync_SetsDependsOnTaskId()
+    {
+        var predecessor = await SeedTaskAsync(TaskStatus.Idle);
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+
+        var result = await _sut.SetDependsOnAsync(task, predecessor, default);
+
+        Assert.True(result.Ok);
+        var t = await GetTaskAsync(task);
+        Assert.Equal(predecessor, t.DependsOnTaskId);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_Null_ClearsDependsOnTaskId_AndWakesQueue()
+    {
+        var predecessor = await SeedTaskAsync(TaskStatus.Idle);
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+        await _sut.SetDependsOnAsync(task, predecessor, default);
+        var wakesBefore = _built.WakeCount();
+
+        var result = await _sut.SetDependsOnAsync(task, null, default);
+
+        Assert.True(result.Ok);
+        var t = await GetTaskAsync(task);
+        Assert.Null(t.DependsOnTaskId);
+        Assert.True(_built.WakeCount() > wakesBefore);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_SelfReference_Rejected()
+    {
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+
+        var result = await _sut.SetDependsOnAsync(task, task, default);
+
+        Assert.False(result.Ok);
+        var t = await GetTaskAsync(task);
+        Assert.Null(t.DependsOnTaskId);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_UnknownDependency_Rejected()
+    {
+        var task = await SeedTaskAsync(TaskStatus.Queued);
+
+        var result = await _sut.SetDependsOnAsync(task, "does-not-exist", default);
+
+        Assert.False(result.Ok);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_NotFound_Rejected()
+    {
+        var predecessor = await SeedTaskAsync(TaskStatus.Idle);
+
+        var result = await _sut.SetDependsOnAsync("does-not-exist", predecessor, default);
+
+        Assert.False(result.Ok);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_DirectCycle_Rejected()
+    {
+        var a = await SeedTaskAsync(TaskStatus.Queued);
+        var b = await SeedTaskAsync(TaskStatus.Queued);
+        var setup = await _sut.SetDependsOnAsync(b, a, default);
+        Assert.True(setup.Ok);
+
+        // a -> b would close the loop a -> b -> a.
+        var result = await _sut.SetDependsOnAsync(a, b, default);
+
+        Assert.False(result.Ok);
+        var t = await GetTaskAsync(a);
+        Assert.Null(t.DependsOnTaskId);
+    }
+
+    [Fact]
+    public async Task SetDependsOnAsync_TransitiveCycle_Rejected()
+    {
+        var a = await SeedTaskAsync(TaskStatus.Queued);
+        var b = await SeedTaskAsync(TaskStatus.Queued);
+        var c = await SeedTaskAsync(TaskStatus.Queued);
+        Assert.True((await _sut.SetDependsOnAsync(b, a, default)).Ok);
+        Assert.True((await _sut.SetDependsOnAsync(c, b, default)).Ok);
+
+        // a -> c would close the loop a -> c -> b -> a.
+        var result = await _sut.SetDependsOnAsync(a, c, default);
+
+        Assert.False(result.Ok);
+    }
+
     // ─── RecoverStaleRunningAsync ─────────────────────────────────────────
 
     [Fact]