Remote compute providers for script and command execution

Once now supports running cacheable scripts and ad-hoc commands on remote compute providers. Add # once remote "microsandbox" or # once remote "daytona" to a script header, or use --remote --compute <provider> with once run and once exec. On a cache miss, stdout and stderr stream back live; on a cache hit, the cached output is replayed and declared outputs are restored. The Daytona adapter connects through the Daytona API using environment variables for the sandbox, API key, workdir, and API URL.