One agent per book, inside the machine that book already had
Each book runs its own agent process inside its own machine — adopted rather than written, and small enough to add no machine.
Hermes is MIT, on PyPI, and runs headless in the interpreter the book image already ships. Measured against a live gateway at roughly 36 MB resident, on machines that have 512 MB — so one agent per book adds no machine and no cost line.
Only the API server runs. No messaging platform is installed, and it could not be useful if it were: a book machine has no public address, so a webhook cannot reach it. That surface belongs to the control plane.