Working directly in the main checkout caused a mid-edit collision today: two concurrent sessions edited the same tree and one reverted the other's in-progress changes. The concurrency bullet now requires every session to enter a git worktree before its first edit and to land work by merging into main only when main is clean at the touched files. Defense: process-only change, no game behavior affected. It serves the constitution's working method ("agents set loose on this repository should be able to build the game ... session after session, without drift") — parallel sessions clobbering each other's working tree is drift at the process level, and the existing "expect the working tree to move under you" guidance proved insufficient in practice.