feat(data): add ExpectsNoDiff flag for verification/audit tasks

An empty diff from a review/audit/verification task looks identical to
one that silently failed to commit. ExpectsNoDiff (TaskEntity, add_task/
update_task, preview_merge_set, review_task via TaskRefDto) lets the
merge helper tell them apart instead of treating every 0-changed-files
result as suspicious.

src/ClaudeDo.Data/Models/TaskEntity.cs

@@ -71,6 +71,10 @@ public sealed class TaskEntity
     // list handler) and the Claude affordances are hidden; a hand-driven ConPTY session is still
     // allowed. New tasks in a manual list default to true.
     public bool IsManual { get; set; }
+    // Set on verification/audit/research tasks whose correct outcome is an empty diff (e.g. "build
+    // and test, don't change code"). Lets the merge-helper session tell that apart from a task that
+    // silently failed to commit -- both look identical to preview_merge_set otherwise.
+    public bool ExpectsNoDiff { get; set; }
     public string? Notes { get; set; }
     public int SortOrder { get; set; }
     public string? SessionSkills { get; set; }