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/broot

@@ -8,14 +8,14 @@ if (command -v broot &>/dev/null); then
     function br {
         local cmd cmd_file code
         cmd_file=$(mktemp)
-        if broot --outcmd "$cmd_file" "$@"; then
-            cmd=$(<"$cmd_file")
-            rm -f "$cmd_file"
-            eval "$cmd"
+        if broot --outcmd "${cmd_file}" "$@"; then
+            cmd=$(<"${cmd_file}")
+            rm -f "${cmd_file}"
+            eval "${cmd}"
         else
             code=$?
-            rm -f "$cmd_file"
-            return "$code"
+            rm -f "${cmd_file}"
+            return "${code}"
         fi
     }
 fi