Reconstruct remote POSIX commands with bounded printf arguments so non-POSIX SSH login shells can forward them without requiring remote base64. Preserve relay and system-SSH stdin, and centralize login-shell flag selection for csh/tcsh compatibility.\n\nValidated against real csh and tcsh OpenSSH targets with built-in and system SSH, including cold relay deployment, stdin upload, PTY I/O, file mutation, and reconnect.
