Switch setup.ts to Alpine rootfs, fix remote deployment

- setup.ts now downloads Alpine Linux minirootfs instead of Ubuntu squashfs
- Installs Alpine packages (openssh, python3, curl, ca-certificates) in chroot
- Fixes install script failing on non-Alpine base rootfs (adduser syntax)
- Clean up unused imports and lint warnings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/snapshot.ts

@@ -13,7 +13,6 @@ import {
 import {
   ensureBaseImage,
   ensureSshKeypair,
-  createRunCopy,
   injectSshKey,
 } from "./rootfs.js";
 import { waitForSsh } from "./ssh.js";