Viper treats an environment variable set to the empty string as unset,
so an empty IRC_LISTEN_ADDR fell back to the :6667 default and the IRC
listener could not be disabled from the environment. The config now
reads IRC_LISTEN_ADDR with os.LookupEnv as well, so an empty value
disables the listener and an unset one keeps :6667. Viper's own switch
for this applies to every setting, so it is not used. New config tests
cover both cases, and README.md says how to disable the listener.
Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
