feat(scenarios,environments): add markdown description field

- add optional description to scenario and environment entities
- expose description in create/update DTOs, MCP tools, and export DTOs
- render description as markdown on detail pages
- add description editor (CodeEditor) to create/edit forms for both resources

server/src/scenario/dto/scenario-export.dto.ts

@@ -46,6 +46,11 @@ export class ScenarioExportDto {
   @IsNotEmpty()
   name: string;
 
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsString()
+  description?: string;
+
   @ApiProperty({ type: [ScenarioStepExportDto] })
   @IsArray()
   @ValidateNested({ each: true })