Preview dependency migrations in isolated scratch directories before committing

When tackling complex dependency bumps (e.g., Astro 6→7, Vite 8) that risk peer-error cascades, the user clones the repo to a /tmp scratch directory, applies temporary overrides to root package.json, and runs pnpm install --no-frozen-lockfile to surface resolution failures — WITHOUT modifying the actual working tree or committing anything. The goal is purely diagnostic: capture peer/dependency conflict output for review before deciding whether to proceed. Follow this pattern for risky migrations: isolate the experiment, never touch the real branch's package.json/lockfile, and present findings to the user before any real change is made.