Agent to Agent: A Shared Inbox for Claude Code

I run more than one Claude Code session at a time. One sits in project-a, one in project-b, one in project-c. There's a fourth, project-d, I keep around just to review the others' work. Each one is great inside its own repo and completely blind to the rest. They share a machine and a user — me — and nothing else. If the session in project-a changed something the others depended on and needed project-b's session to act on it, the only wire between them was me, copying a sentence from one terminal into another.