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/agent-manager.ts

@@ -1,4 +1,4 @@
-import { spawn, type ChildProcess } from "node:child_process";
+import { spawn } from "node:child_process";
 import {
   existsSync,
   mkdirSync,