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/anyenv

@@ -1,6 +1,6 @@
 # https://github.com/riywo/anyenv
 
-# git clone https://github.com/riywo/anyenv $HOME/.anyenv
+# git clone https://github.com/riywo/anyenv ${HOME}/.anyenv
 
 if (which anyenv &>/dev/null); then
   eval "$(anyenv init -)"