fix: publish importable DRIFT package

[tuzuminami]

Closes #10

What changed

• adds explicit root, server, and migration package exports with declaration paths
• verifies a packed tarball in a clean consumer install
• verifies public imports, CLI symlink execution, and private subpath rejection
• fixes CLI entrypoint detection through npm symlinked bin paths

Verification

• pnpm install --frozen-lockfile
• npm run check (36 passing; PostgreSQL integration skipped locally without DRIFT_POSTGRES_TEST_URL)
• packed tarball manifest contains only the intended distribution surface