fix: rename app back to default.json — appId must be 'default'

The importer uses the filename as appId, and the frontend only queries
for appId='default'. Renaming to cortex.json silently created the app
with the wrong ID.

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

Author: deanzaka

README.md

@@ -102,7 +102,7 @@ cortex/
 │   └── monday-planning.json
 │
 ├── apps/
-│   └── cortex.json             Genesis app (React SPA)
+│   └── default.json            Genesis app (React SPA, must be "default")
 │
 ├── examples/                   Blank templates for creating new artifacts
 │   ├── agent-template.json