Run scripts and targets on remote compute providers

The once exec and once run commands can now dispatch actions to a remote compute provider. Use the --remote flag and select a provider with --compute (defaulting to microsandbox). When running remotely, stdout and stderr are streamed back live while still being captured for caching. Remote compute is gated to Unix platforms and the embedded microsandbox provider is limited to Linux and Apple Silicon macOS, avoiding unsupported Intel macOS dependency paths.