check Stripe payments against a policy: [marcosgcunha]

[marcosgcunha]

What do you want to work on?

I want to add a supervisor middleware example that checks Stripe payments against a policy inside the sandbox. I think it belongs in the examples folder because most agent money losses happen with no attacker and the sandbox boundary sees the host but not the amount. The example is self-contained with tests. I opened it as #3919 and it was auto-closed. Thank you

Why this change?

I run a company that builds a policy gate for agent payments (Axiru.com), so I've spent months looking at how agents lose money. Almost every case I've seen is a mistake, not an attack. It's a retry that refunds twice, or a parsing error that sends the whole balance. OpenShell is the right place to catch that because it already sits between the agent and the network. It just needs to see the amount so I'd like the example to show how.

Checklist

• I wrote this request myself (not AI-generated)
• I have read CONTRIBUTING.md