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

client/src/pages/scenario/ScenarioDetailPage.tsx

@@ -37,6 +37,7 @@ import {
   Table,
   Timestamp,
   UuidBadge,
+  MarkdownContent,
   useToast,
   type TableColumn,
 } from '../../ui';
@@ -400,6 +401,17 @@ export function ScenarioDetailPage() {
             />
           </Card>
 
+          {scenario.description && (
+            <div className={styles.stepsSection}>
+              <div className={styles.sectionHeadingRow}>
+                <h2 className={styles.sectionHeading}>{t('scenarios.field_description')}</h2>
+              </div>
+              <Card>
+                <MarkdownContent content={scenario.description} />
+              </Card>
+            </div>
+          )}
+
           {scenario.steps.length > 0 && (
             <div className={styles.stepsSection}>
               <div className={styles.sectionToolbar}>