Keep development data in .devenv

Keep development data in .devenv

The development shell sets ONECHAT_DIR to .devenv/onechat at the root of
the checkout, unless the variable is already set. Development data, such
as a copy of an old chat log for a replay, stays inside the checkout,
away from a running service, and Git ignores it. Each worktree has its
own .devenv, so parallel work does not share data.