Let an empty IRC_LISTEN_ADDR disable the IRC listener (closes #124)

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>

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 import (
 	"errors"
 	"log/slog"
+	"os"
 
 	"github.com/spf13/viper"
 	"go.uber.org/fx"
@@ -129,6 +130,13 @@ func New(
 		params:             &params,
 	}
 
+	// Viper counts an empty environment variable as unset, but an
+	// empty IRC_LISTEN_ADDR must turn the IRC listener off.
+	ircListenAddr, set := os.LookupEnv("IRC_LISTEN_ADDR")
+	if set {
+		cfg.IRCListenAddr = ircListenAddr
+	}
+
 	if cfg.Debug {
 		params.Logger.EnableDebugLogging()
 		cfg.log = params.Logger.Get()