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/Migrations/ClaudeDoDbContextModelSnapshot.cs

@@ -512,6 +512,12 @@ namespace ClaudeDo.Data.Migrations
                         .HasColumnType("TEXT")
                         .HasColumnName("description");
 
+                    b.Property<bool>("ExpectsNoDiff")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER")
+                        .HasDefaultValue(false)
+                        .HasColumnName("expects_no_diff");
+
                     b.Property<int?>("FailureMaxTurns")
                         .HasColumnType("INTEGER")
                         .HasColumnName("failure_max_turns");