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.

libs/todo.bash

@@ -1,20 +1,20 @@
 todo() {
-    if [[ ! -f $HOME/.todo ]]; then
-        touch $HOME/.todo
+    if [[ ! -f "${HOME}/.todo" ]]; then
+        touch "${HOME}/.todo"
     fi
 
     if [[ $# -eq 0 ]]; then
-        cat $HOME/.todo
+        cat "${HOME}/.todo"
     elif [[ "$1" == "-l" ]]; then
-        cat -n $HOME/.todo
+        cat -n "${HOME}/.todo"
     elif [[ "$1" == "-c" ]]; then
-        echo "" > $HOME/.todo
+        echo "" > "${HOME}/.todo"
     elif [[ "$1" == "-r" ]]; then
-        cat -n $HOME/.todo
+        cat -n "${HOME}/.todo"
         echo -ne "----------------------------\nType a number to remove: "
         read NUMBER
-        sed -ie ${NUMBER}d $HOME/.todo
+        sed -ie "${NUMBER}d" "${HOME}/.todo"
     else
-        echo "$@" >> $HOME/.todo
+        echo "$@" >> "${HOME}/.todo"
     fi
 }