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/note.bash

@@ -1,17 +1,17 @@
 note() {
     # if file doesn't exist, create it
-    if [[ ! -f $HOME/.notes ]]; then
-        touch "$HOME"/.notes
+    if [[ ! -f "${HOME}/.notes" ]]; then
+        touch "${HOME}"/.notes
     fi
 
     if [[ $# -eq 0 ]]; then
         # no arguments, print file
-        cat "$HOME"/.notes
+        cat "${HOME}"/.notes
     elif [[ "$1" == "-c" ]]; then
         # clear file
-        echo "" >"$HOME"/.notes
+        echo "" >"${HOME}"/.notes
     else
         # add all arguments to file
-        echo "$@" >>"$HOME"/.notes
+        echo "$@" >>"${HOME}"/.notes
     fi
 }