Neutralizes inherited startup-file hooks so a non-login/non-interactive
sh -c / bash -c cannot be hijacked into sourcing a profile or rc file
(#451). lean-ctx runs the system POSIX shell profile-free and deterministic;
a stray inherited BASH_ENV (read by bash even for bash -c) or ENV
pointing at e.g. an exec nu snippet would otherwise silently replace the
shell. Cleared to empty so the expansion names no file. No-op in effect for
PowerShell/cmd, which ignore both variables.
Sets LC_CTYPE=C.UTF-8 when no UTF-8 locale is inherited from the parent
process. Without this, commands treat bytes >127 as non-printable (C locale),
mangling Cyrillic, CJK, emoji, etc.