feat(example): optional pre-payment policy guard for x402 MCP client

Hi maintainers — filing before a PR per CONTRIBUTING.

Would an opt-in policy check fit the existing `examples/x402-mcp` example?

The draft composes with the existing `confirmationCallback`:

1. existing human confirmation
2. optional plain-fetch IntentFence preview
3. sign only on `safe_to_proceed`

It is disabled unless `INTENTFENCE_PREFLIGHT_URL` is set, adds no dependency, and fails closed only after explicit opt-in. It sends the selected x402 network, recipient, asset, and USDC amount, and clearly documents that the free preview is advisory and does not prove authorization or settlement.

Draft: https://github.com/razel369/agents/tree/codex/intentfence-x402-preflight/examples/x402-mcp

Validation: 6 tests, TypeScript, focused lint/format, and Vite build.

If this product-specific URL is not suitable upstream, I am happy to generalize the hook or keep it as an external integration. Is this in scope for a PR?