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/External/BatchMcpToolsTests.cs

@@ -216,6 +216,23 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.Equal("the full description", found.TaskFull!.Description);
     }
 
+    [Fact]
+    public async Task BatchGetTasks_QueuedWithUnmetDependsOn_ReportsBlocked()
+    {
+        var listId = await SeedListAsync();
+        var predecessor = await SeedTaskAsync(listId, "predecessor", TaskStatus.Idle);
+        var task = await SeedTaskAsync(listId, "blocked", TaskStatus.Queued);
+        task.DependsOnTaskId = predecessor.Id;
+        await _tasks.UpdateAsync(task);
+        var sut = BuildSut();
+
+        var results = await sut.BatchGetTasks(new[] { task.Id }, cancellationToken: CancellationToken.None);
+
+        var found = results.Single(r => r.Id == task.Id);
+        Assert.True(found.Task!.Blocked);
+        Assert.Contains(predecessor.Id, found.Task!.BlockedReason);
+    }
+
     [Fact]
     public async Task BatchDeleteTasks_RunningTask_ReportedNotOk_OthersDeleted()
     {