computer sandbox

Authorization: Bearer <SANDBOX_TOKEN> is required on /c routes.

  GET  /c/<name>                         list backends
  PUT  /c/<name>/file/workspace/<path>   write file at /workspace/<path>
  GET  /c/<name>/file/workspace/<path>   read file at /workspace/<path>
  POST /c/<name>/fs                      atomic filesystem & search RPC
  POST /c/<name>/exec                    run a command (JSON result)

exec body: { "command" | "argv", "cwd"?, "env"?, "backend"?: "shell" | "container" }
backend defaults to shell. container is a Linux userland and starts on first use.