anona-compute exec

Run a command in a running sandbox and stream its output. anona-compute exits with
the command's exit code. With one argument the command is a shell string
(anona-compute exec ID -- 'ls | wc -l'); several arguments are quoted and joined.

With --json, output is one object per line: {"type":"stdout"|"stderr","data":...}
and finally {"type":"exit","exitCode":N}.

anona-compute exec SANDBOX_ID -- COMMAND [ARGS...] [flags]