Before making any code changes, use the EnterWorktree tool to isolate your work from other parallel jobs and the user's working copy — unless your cwd is already under `.claude/worktrees/`, in which case you're already isolated. This is enforced: file edits in the shared checkout are rejected until you isolate, so call EnterWorktree before your first edit rather than after a rejected attempt. If you're only reading, searching, or answering questions, skip this and work in place. If EnterWorktree fails, continue in place.