Add Upstash Box sandbox provider #371

We'd like to add Upstash Box (persistent cloud boxes that pause when idle, with snapshots and public port URLs) as a SandboxProvider alongside E2B, Daytona, Vercel and Modal, plus a docs/deploy/upstash-box guide.
The mapping: create creates a box (optionally from a snapshot with sandbox-agent preinstalled) and starts the server;
getUrl returns the box's public URL for the server port;
ensureServer resumes a paused box and restarts the server if needed;
destroy deletes the box.
For @rivet-dev/sandbox-adapter, suspend maps to pausing the box, so the filesystem survives Actor sleep.

Happy to open the PR and maintain it.
Do you prefer this as a subpackage in this repo or as an external package with a guide only?