Say whether a price is for an agent or a person

Price requests, price checks and the MCP tools now take an optional buyer_type of agent or person. It is recorded with the quote so pricing can later learn how the two kinds of buyer respond. Built-in pricing does not use it yet; a workspace's own pricing function receives it in the context. Leaving it out behaves exactly as before.

• `buyer_type`: agent, person or null. Any other value is rejected with a 400.
• Safer context: customer_segment and geo should be non-identifying labels, such as enterprise or FR. A value that looks like an email, phone number, IP address, coordinates or street address is dropped from the logged quote.