fix(fish): guard nvim aliases and add eza fallbacks

Guard vim/vi → nvim aliases with type -q nvim check so they only
activate when nvim is installed. Add ls/ll/l/lsa fallbacks when eza
is not installed. Add brew shellenv fish eval in config.fish.

config/fish/config.fish

@@ -52,3 +52,4 @@ source ~/.orbstack/shell/init2.fish 2>/dev/null || :
 if status is-interactive; and not set -q GITHUB_TOKEN
     echo "Warning: GITHUB_TOKEN is not set" >&2
 end
+eval "$(/opt/homebrew/bin/brew shellenv fish)"