POSIX shell rendering for the few HostKit paths that must cross sh -c or SSH exec.
Prefer structured argv execution (System.cmd/3, HostKit.Resources.Command.exec) whenever possible.
Elixir's System.cmd/3 passes arguments as argv and explicitly does not require shell
quoting. This module exists only for boundaries that are inherently shell strings, such as
remote SSH command lines and package-manager probes that use shell operators.