fix: emit named exports in generated env configs - #20

Problem

A fresh pnpm create cellajs/cella scaffold fails its first pnpm generate:

SyntaxError: The requested module '../../config/config.development' does not provide an export named 'development'

Cella's config-builder/app-config.ts now imports each environment config by name (import { development } from '../../config/config.development') and config.default via the named config export, but generateEnvConfigs still emitted the old export default shape with import type _default from './config.default'.

Fix

• Emit export const <mode> = { ... } for every environment file
• Import the default config type as import type { config as _default } from './config.default'
• Use the named { development } import in the generated test config
• Header before extra imports, matching cella's file layout

Tests

• Added a regression test asserting every generated mode file uses the named-export shape (and contains no export default)
• Updated the test-config import assertion
• Verified end-to-end: patched an affected scaffold's config files the same way and pnpm generate runs clean

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