feat(scenarios): overhaul export/import with multi-entity YAML format

- export now accepts includeEnvironment and credentialIds query params
- output is a YAML stream with comment-delimited environment, credential, and scenario sections
- scenario entity includes credentials array with alias mappings
- import accepts both old single-object and new array format, upserts envs/creds before linking
- new ExportScenarioModal with env and per-credential checkboxes replaces direct download

client/src/api/client.ts

@@ -216,8 +216,19 @@ export const scenarios = {
   ): Promise<PaginatedResponse<ScenarioRun & { stepRuns: ScenarioRunStep[] }>> {
     return request(`/scenarios/${scenarioId}/runs?page=${page}&limit=${limit}`);
   },
-  exportScenario(id: string): Promise<string> {
-    return request(`/scenarios/${id}/export`);
+  exportScenario(
+    id: string,
+    options: { includeEnvironment: boolean; credentialIds: string[] } = {
+      includeEnvironment: false,
+      credentialIds: [],
+    },
+  ): Promise<string> {
+    const params = new URLSearchParams();
+    params.set('includeEnvironment', String(options.includeEnvironment));
+    for (const cid of options.credentialIds) {
+      params.append('credentialIds', cid);
+    }
+    return request(`/scenarios/${id}/export?${params.toString()}`);
   },
   importScenario(payload: unknown): Promise<Scenario> {
     return request('/scenarios/import', {