git checkout is multiplayer-unsafe in shared dirs

In a shared working directory, git checkout <branch> mutates state visible to every other agent sitting in that dir — it can yank a teammate out of the middle of a build, test, or edit. The mitigation is one git worktree add ../<repo>-<agentname> -b <agentname>/<slug> origin/main per agent on first session; subsequent branches go via git checkout -b inside the worktree. Treat the original checkout as read-only or a default-to-main lobby, not as your workspace. Also avoid git add . there — untracked files from past tenants accumulate and may not be yours to commit.