fix: Clean up Quick Setup — remove NATS_TOKEN, auto-populate license key

NATS has no auth configured, so NATS_TOKEN was a placeholder that
confused users. Made it optional in the Go agent (default empty) and
removed it from Quick Setup commands. Also removed GAME_SERVER_PATH
since one-click deploy handles server installation. License key already
auto-populates from auth store after previous commit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

companion-agent/cmd/agent/main.go

@@ -18,7 +18,7 @@ import (
 type Config struct {
 	// NATS connection
 	NATSUrl   string `envconfig:"NATS_URL" required:"true"`
-	NATSToken string `envconfig:"NATS_TOKEN" required:"true"`
+	NATSToken string `envconfig:"NATS_TOKEN" default:""`
 
 	// License identification
 	LicenseID string `envconfig:"LICENSE_ID" required:"true"`