fix: include default-flows JSON in nest build assets

nest build only compiles TS — JSON files need explicit asset copy
in nest-cli.json compilerOptions.assets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nest-cli.json

@@ -3,6 +3,9 @@
   "collection": "@nestjs/schematics",
   "sourceRoot": "src",
   "compilerOptions": {
-    "deleteOutDir": true
+    "deleteOutDir": true,
+    "assets": [
+      { "include": "messaging/flow/default-flows/*.json", "watchAssets": true }
+    ]
   }
 }