style: Normalize variable expansions to ${var} across the repo

Replace bare $var with ${var} everywhere it was still used, matching
the convention already established in rc.d/prompt and rc.d/history.
Positional parameters ($1, $@, $#) and special variables ($?, $$, $-)
are left as-is, consistent with existing usage in _helpers.bash.

rc.d/broot is auto-generated by the broot binary and will likely be
overwritten with its original formatting on the next broot update.

rc.d/screen

@@ -1,7 +1,7 @@
 # GNU Screen stuff
 if ( which screen &> /dev/null )
 then
-  if [[ $TERM != screen* ]]
+  if [[ ${TERM} != screen* ]]
   then
     if [ "$(screen -ls | sed -ne 's/[[:space:]]//' -ne 's/(Attached)// p')" ]
     then