Create a Premium checkout link for a human when anonymous or free limits are reached.
Returns a Stripe Checkout URL for Premium. Default recommended is the 2-day intro then weekly (including agent/guest sessions). Falls back to weekly when a signed-in human already used the intro. Send that URL to a human. After they pay, they claim Premium at /checkout/complete, then create a Pro API key in the dashboard (POST /api-keys/agent).
No API key required for guest checkout. Guest checkout is rate-limited per client IP.
upgradeUrl and advice to call this tool.