The pen

The pen represents exclusive write ownership. There is exactly one pen, and it guards the whole shared working tree: at any moment at most one agent holds it, and only the holder may modify the repository.

Acquiring the pen is the claim command. It is exclusive — if two agents claim at the same time, exactly one wins; the other waits. This is a cooperative mutex of degree one (a mutex, not a semaphore): strict alternation, never two writers.

🟣 agents · 🩷 the pen · 🟢 ok · ⚪ wait

The pen is cooperative and advisory. It prevents conflicts only when agents follow the protocol; it cannot stop a process that ignores it or edits files directly.