Coordinating Two Sessions on One Repo via Cross-Session Messaging

Two of your Claude Code sessions are working the same repository in separate git worktrees: session A is refactoring a shared PaymentProcessor interface, session B is building a new feature on top of the current interface. You want A to warn B the moment its interface change lands, so B doesn't keep building against a signature that's about to disappear.

1. Design how you'd set this up — what you'd ask Claude to do, and when.
2. Walk through exactly what session B's Claude is and is not allowed to do with the message it receives from A, and why those limits exist.
3. What trust boundary would you additionally want to configure if session A and session B ran on two different physical machines instead of two worktrees on the same one?