fix(worker): carry ExpectsNoDiff into the relocated merge MCP tools

Conflict resolution for the #266 merge: #247 had moved PreviewMerge/PreviewMergeSet
and MergePreviewSetEntryDto out of ExternalMcpService into MergeMcpTools, so #266's
in-place edits to those had to be re-applied at the new location, together with the
PreviewMergeSet_ReportsExpectsNoDiffPerTask test.

src/ClaudeDo.Worker/External/MergeMcpTools.cs

@@ -57,7 +57,11 @@ public sealed record MergePreviewToolDto(
 public sealed record MergePreviewSetEntryDto(
     string TaskId, string Status, IReadOnlyList<string> ConflictFiles, int ChangedFileCount, int Behind, string? Error, bool IsEmpty = false,
     int? VerifyExitCode = null, long? VerifyDurationMs = null, string? VerifyOutputTail = null,
-    IReadOnlyList<string>? StaleFiles = null, int? Number = null);
+    IReadOnlyList<string>? StaleFiles = null, int? Number = null,
+    // Mirrors TaskEntity.ExpectsNoDiff -- lets the merge helper tell a deliberately empty diff
+    // (verification/audit/research task) apart from a task that silently failed to commit, both
+    // of which otherwise show IsEmpty=true with no other distinguishing signal.
+    bool ExpectsNoDiff = false);
 
 public sealed record FileOverlapDto(string File, IReadOnlyList<string> TaskIds, IReadOnlyList<int> Numbers);
 
@@ -461,7 +465,7 @@ public sealed class MergeMcpTools
         IProgress<ProgressNotificationValue>? progress = null)
     {
         taskId = await TaskIdResolver.ResolveAsync(_tasks, taskId, cancellationToken);
-        var (preview, behind, _, isEmpty, staleFiles, _) = await PreviewMergeCoreAsync(taskId, targetBranch, runVerify: true, cancellationToken, progress);
+        var (preview, behind, _, isEmpty, staleFiles, _, _) = await PreviewMergeCoreAsync(taskId, targetBranch, runVerify: true, cancellationToken, progress);
         return new MergePreviewToolDto(preview.Status, preview.ConflictFiles, preview.ChangedFileCount, behind, isEmpty,
             preview.VerifyExitCode, preview.VerifyDurationMs, preview.VerifyOutputTail, staleFiles);
     }
@@ -503,10 +507,10 @@ public sealed class MergeMcpTools
             var taskId = taskIds[i];
             try
             {
-                var (preview, behind, changedFiles, isEmpty, staleFiles, number) = await PreviewMergeCoreAsync(taskId, targetBranch, runVerify, cancellationToken, progress);
+                var (preview, behind, changedFiles, isEmpty, staleFiles, number, expectsNoDiff) = await PreviewMergeCoreAsync(taskId, targetBranch, runVerify, cancellationToken, progress);
                 entries.Add(new MergePreviewSetEntryDto(
                     taskId, preview.Status, preview.ConflictFiles, preview.ChangedFileCount, behind, null, isEmpty,
-                    preview.VerifyExitCode, preview.VerifyDurationMs, preview.VerifyOutputTail, staleFiles, number));
+                    preview.VerifyExitCode, preview.VerifyDurationMs, preview.VerifyOutputTail, staleFiles, number, expectsNoDiff));
                 filesByTask[taskId] = changedFiles;
                 numbersByTask[taskId] = number;
             }
@@ -515,7 +519,7 @@ public sealed class MergeMcpTools
                 var task = await _tasks.GetByIdAsync(taskId, cancellationToken);
                 entries.Add(new MergePreviewSetEntryDto(
                     taskId, TaskMergeService.PreviewUnavailable, Array.Empty<string>(), 0, 0, ex.Message,
-                    Number: task?.Number));
+                    Number: task?.Number, ExpectsNoDiff: task?.ExpectsNoDiff ?? false));
             }
 
             ProgressReporter.ReportItem(progress, "Previewing merges", i + 1, taskIds.Count);
@@ -573,7 +577,7 @@ public sealed class MergeMcpTools
     // (its commits already sit on the list's working dir) — falls back to the fixed
     // HandlerBaseCommit..HandlerHeadCommit range, reporting a synthetic "clean" preview of that
     // range's own diff-stat instead of throwing "has no worktree".
-    private async Task<(MergePreviewResult Preview, int Behind, IReadOnlyList<string> ChangedFiles, bool IsEmpty, IReadOnlyList<string> StaleFiles, int Number)> PreviewMergeCoreAsync(
+    private async Task<(MergePreviewResult Preview, int Behind, IReadOnlyList<string> ChangedFiles, bool IsEmpty, IReadOnlyList<string> StaleFiles, int Number, bool ExpectsNoDiff)> PreviewMergeCoreAsync(
         string taskId, string? targetBranch, bool runVerify, CancellationToken ct,
         IProgress<ProgressNotificationValue>? progress = null)
     {
@@ -615,7 +619,7 @@ public sealed class MergeMcpTools
                 .Intersect(targetChangedSinceFork, StringComparer.OrdinalIgnoreCase)
                 .ToList();
 
-            return (preview, behind, changedFiles, changedFiles.Count == 0, staleFiles, task.Number);
+            return (preview, behind, changedFiles, changedFiles.Count == 0, staleFiles, task.Number, task.ExpectsNoDiff);
         }
 
         if (task.HandlerBaseCommit is { Length: > 0 } handlerBase && task.HandlerHeadCommit is { Length: > 0 } handlerHead)
@@ -632,7 +636,7 @@ public sealed class MergeMcpTools
             // No fork point to diff against: a handler task commits straight onto the list's
             // working dir instead of a branch, so there is nothing else that could have "changed
             // in the target since the fork".
-            return (preview, 0, changedFiles, isEmpty, Array.Empty<string>(), task.Number);
+            return (preview, 0, changedFiles, isEmpty, Array.Empty<string>(), task.Number, task.ExpectsNoDiff);
         }
 
         throw new InvalidOperationException($"Task {taskId} has no worktree.");

tests/ClaudeDo.Worker.Tests/External/MergeMcpToolsTests.cs

@@ -1139,6 +1139,22 @@ public sealed class MergeMcpToolsTests : IDisposable
         Assert.Empty(result.Overlaps);
     }
 
+    [Fact]
+    public async Task PreviewMergeSet_ReportsExpectsNoDiffPerTask()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var (task, list, _) = await SeedWorktreeAsync(TaskStatus.WaitingForReview);
+        task.ExpectsNoDiff = true;
+        await _tasks.UpdateAsync(task, CancellationToken.None);
+        var target = await new GitService().GetCurrentBranchAsync(list.WorkingDir!, CancellationToken.None);
+        var sut = BuildSut(CreateQueue());
+
+        var result = await sut.PreviewMergeSet([task.Id], target, cancellationToken: CancellationToken.None);
+
+        Assert.True(Assert.Single(result.Tasks).ExpectsNoDiff);
+    }
+
     [Fact]
     public async Task PreviewMergeSet_ReportsProgressPerTask()
     {