fix(install): provide default for SHELL variable

Use ${SHELL:-sh} to prevent script crash when $SHELL is unset
in minimal containers, CI/CD runners, or non-interactive shells
where set -u is enabled.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: betegon

docs/public/install

@@ -114,7 +114,7 @@ add_to_path() {
 
 XDG_CONFIG_HOME=${XDG_CONFIG_HOME:-$HOME/.config}
 
-current_shell=$(basename "$SHELL")
+current_shell=$(basename "${SHELL:-sh}")
 case $current_shell in
   fish)
     config_files=("$HOME/.config/fish/config.fish")