OPP: Cross-sandbox and multi-agent flow verification #1056

Extend the policy prover to reason about permitted flows across multiple connected sandboxes, not just individual sandbox policies in isolation.

Problem

Today OPP verifies a single sandbox policy at a time. In real deployments, agents in separate sandboxes communicate with each other -- one sandbox posts to Slack, another reads from GitHub, a third aggregates and reports to Telegram. The security question becomes: what is the combined allowed flow across the system?

A policy that looks locked down in isolation may still enable unintended end-to-end paths when composed with other sandboxes. For example:

• Sandbox A can read from a database and post to Slack
• Sandbox B can read from Slack and push to GitHub
• Together, data flows from the database to GitHub even though neither sandbox individually allows that

Scope

• Accept multiple policy files as input (one per sandbox)
• Model shared endpoints as connection points between sandboxes
• Run reachability analysis across the composed graph
• Surface end-to-end exfiltration and write paths that only exist through composition
• Report which sandbox boundaries the flow crosses

Outcome

Users can answer: "given these N sandboxes and their policies, what end-to-end data flows are possible?" This is especially relevant as multi-agent architectures become more common.

Context

Tracks under OS-43. This is the most forward-looking expansion item -- depends on single-sandbox verification being solid first.