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/asdf

@@ -8,7 +8,7 @@ if [ -f ~/.asdf/asdf.sh ]; then
 
 fi
 
-if [ -f "$HOME/.nix-profile/share/asdf-vm/asdf.sh" ]; then
-	. "$HOME/.nix-profile/share/asdf-vm/asdf.sh"
-	. "$HOME/.nix-profile/share/bash-completion/completions/asdf.bash"
+if [ -f "${HOME}/.nix-profile/share/asdf-vm/asdf.sh" ]; then
+	. "${HOME}/.nix-profile/share/asdf-vm/asdf.sh"
+	. "${HOME}/.nix-profile/share/bash-completion/completions/asdf.bash"
 fi