fix(examples/integrations/langgraph-js): remove stale top-level app/ dir shadowing src/app home route

The starter had two App Router dirs: a stale top-level app/ containing only
an /api/copilotkit route (with a wrong graphId "starterAgent") and the real
src/app/ with page.tsx + layout.tsx. At copilotkit 1.59.3 the build silently
dropped the / route from src/app (masked by next.config ignoreBuildErrors),
producing only /api/copilotkit/[[...slug]]. The app served GET / as 404, so the
docker healthcheck (GET / === 200) never passed and the smoke leg failed with
the app container unhealthy before tests ran.

Removing the duplicate top-level app/ lets Next build src/app unambiguously,
restoring the / route. Local docker-compose.test.yml smoke: app healthy in ~5s,
4 passed (@health, @agent, @chat, @Interaction).

Author: jpr5