v0.8.0

Added

• @p2flux/sdk/paywall — charge AI agents for a route over x402 without an x402 library:
  createPaywall({ apiUrl, recipient, price }) with express() middleware, wrap() for Fetch-API
  handlers, and the framework-neutral guard(). Pay-per-request and prepaid balance; the payment is
  settled before the handler runs and serves one response. A separate entry point: the main client
  is unchanged and byte-identical.
• Usage pricing — paywall.usage({ url, paymentHeader, maxPrice }, work): the agent signs for at
  most maxPrice (x402 upto), you charge what the request cost.
• Requests signed as bots (Web Bot Auth) count as agents under agentsOnly; an agent's prepaid
  refund request is answered with its receipt.