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/tmux

@@ -1,7 +1,7 @@
 # GNU Screen stuff
 if (command -v tmux &>/dev/null); then
-  if [ "$(pgrep -u "$UID" "tmux" | wc -l)" -gt 0 ]; then
-    if [[ $TERM != screen* ]]; then
+  if [ "$(pgrep -u "${UID}" "tmux" | wc -l)" -gt 0 ]; then
+    if [[ ${TERM} != screen* ]]; then
       nb_sessions=$(tmux list-session | wc -l)
       nb_sessions_attached=$(tmux list-session | grep -c "attached")
       nb_sessions_detached=$(tmux list-session | grep -c -v "attached")