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/krew

@@ -1,3 +1,3 @@
 if (command -v krew &>/dev/null); then
-	export PATH="${KREW_ROOT:-$HOME/.krew}/bin:$PATH"
+	export PATH="${KREW_ROOT:-${HOME}/.krew}/bin:${PATH}"
 fi