feat(api): add assigned_agent to Task model (MS22-DB-003, MS22-API-003)

apps/api/src/tasks/dto/update-task.dto.ts

@@ -52,6 +52,12 @@ export class UpdateTaskDto {
   @IsUUID("4", { message: "parentId must be a valid UUID" })
   parentId?: string | null;
 
+  @IsOptional()
+  @IsString({ message: "assignedAgent must be a string" })
+  @MinLength(1, { message: "assignedAgent must not be empty" })
+  @MaxLength(255, { message: "assignedAgent must not exceed 255 characters" })
+  assignedAgent?: string | null;
+
   @IsOptional()
   @IsInt({ message: "sortOrder must be an integer" })
   @Min(0, { message: "sortOrder must be at least 0" })