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/20260902102003_AddExpectsNoDiff.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ClaudeDo.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddExpectsNoDiff : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<bool>(
+                name: "expects_no_diff",
+                table: "tasks",
+                type: "INTEGER",
+                nullable: false,
+                defaultValue: false);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "expects_no_diff",
+                table: "tasks");
+        }
+    }
+}