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.

functions.d/devenv

@@ -9,7 +9,7 @@ function devenv() {
   if [ "${status}" != "RUNNING" ]; then lxc start devenv; fi
 
   # Rename tmux window if inside tmux
-  if [ -n "$TMUX" ]; then tmux rename-window "devenv"; fi
+  if [ -n "${TMUX}" ]; then tmux rename-window "devenv"; fi
 
   # Login as user xavier
   lxc exec devenv -- login -f xavier