fix(sandbox): keep app runtimes off persistent storage

[iamjr15]

What

• split Next and Expo source-only scaffolds from immutable local dependency runtimes
• keep dependency trees and Next compiler output off Daytona object-store FUSE
• add snapshot smoke checks for source/runtime separation

Why

Production tracing showed the app-builder timing out before model invocation while copying roughly 400 MB of pnpm data into the persistent workspace. The timed-out FUSE copy left partial projects and made retries fail inconsistently.

Verification

• pnpm typecheck
• pnpm lint
• pnpm turbo build --force
• pnpm architecture:check
• pnpm deadcode
• isolated pnpm install wrote the complete Next graph to an external modules directory in 1.6s and left the project without node_modules

Rollout

This PR intentionally publishes only the snapshot-side prerequisite. After merge, build the immutable Daytona candidate from this exact main commit. Worker promotion follows in a separate reviewed PR that pins that candidate, avoiding any interval where production code expects a runtime absent from the active snapshot.