Relay INVITE to IRC listener users as an INVITE line (closes #147)

An invite for an IRC listener user arrived as a server NOTICE that did
not name the inviter, so no IRC client treated it as an invite. The
relay now sends it as an INVITE line from the inviter, with the channel
taken from the message body, and sends nothing when the body names no
channel. The IRC listener now queues the INVITE with the channel as its
body, as the HTTP API already did, so an invite from either transport
has the same shape. The README's IRC listener section says how an
invite arrives.

Model: opus-5-5

internal/ircserver/commands.go

@@ -1261,10 +1261,9 @@ func (c *Conn) handleInvite(
 		irc.RplInviting, targetNick, channel,
 	)
 
-	// Send INVITE notice to target via service fan-out.
-	body, _ := json.Marshal( //nolint:errchkjson
-		[]string{"You have been invited to " + channel},
-	)
+	// Queue the INVITE to the target with the body the HTTP
+	// API uses.
+	body, _ := json.Marshal([]string{channel}) //nolint:errchkjson
 
 	_, _, _ = c.svc.FanOut( //nolint:dogsled // fire-and-forget broadcast
 		ctx, "INVITE", c.nick, targetNick, targetID, 0,