How agent billing consent should work

Agent billing should be explicit. When an unauthenticated or free account reaches the paid investigation path, MSX tells the agent Pro is required before live market discovery can run.

The agent should ask the user whether they want to subscribe. Only after the user agrees should it request a Stripe Checkout URL from the API.

Payment is tied to the account

The checkout flow links Stripe state back to the MSX account created through device auth. After Stripe confirms the subscription, the same account-bound API key can continue with the investigation.

This preserves the agent flow without asking the user to copy private billing state into chat.