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.

profile.d/anyenv

@@ -1,8 +1,8 @@
 # https://github.com/riywo/anyenv
 
-# git clone https://github.com/riywo/anyenv $HOME/.anyenv
+# git clone https://github.com/riywo/anyenv ${HOME}/.anyenv
 
-if [ -d "$HOME/.anyenv/bin" ]
+if [ -d "${HOME}/.anyenv/bin" ]
 then
-  export PATH="$HOME/.anyenv/bin:$PATH"
+  export PATH="${HOME}/.anyenv/bin:${PATH}"
 fi