Follow-up to #758

Problem

First nuxi prepare fails on fresh project when .nuxt/ doesn't exist.

[UNHANDLEABLE_ERROR] Error: Tsconfig not found .nuxt/tsconfig.json
Caused by: Failed to resolve tsconfig option

PR #758 enabled schema building during prepare, but rolldown tries to resolve the project's tsconfig.json which extends .nuxt/tsconfig.json - a file that doesn't exist yet.

Fix

Add tsconfig: false at the top level of tsdown build options. This prevents rolldown from resolving the tsconfig chain while still generating .d.mts types.

StackBlitz

CLI Reproduction

git clone --depth 1 --filter=blob:none --sparse https://github.com/onmax/repros.git
cd repros && git sparse-checkout set nuxthub-758
cd nuxthub-758 && pnpm i && pnpm prepare  # fails

Verify fix

git sparse-checkout add nuxthub-758-fixed
cd ../nuxthub-758-fixed && pnpm i && pnpm prepare  # succeeds
ls .nuxt/hub/db/  # schema.mjs + schema.d.mts exist