Add ensureServer() to SandboxProvider interface to handle cases where the
sandbox-agent server stops or goes to sleep. The SDK now calls this method
after 3 consecutive health-check failures, allowing providers to restart the
server if needed. Most built-in providers (E2B, Daytona, Vercel, Modal,
ComputeSDK) implement this. Docker and Cloudflare manage server lifecycle
differently, and Local uses managed child processes.
Also update docs for quickstart, architecture, multiplayer, and session
persistence; mark persist-* packages as deprecated; and add ensureServer
implementations to all applicable providers.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
