Forked subagents can no longer exit plan mode

Plan mode is a mode where Claude plans its work for your approval before making changes. A forked subagent is a helper that is started as a copy of the current session to handle part of the work. Such a subagent now cannot use the ExitPlanMode tool, whether it is in plan mode or in the fallback mode. It gets the message "A fork cannot exit plan mode; that belongs to the session that forked it. Finish your part and report back." Before, the only check was whether the session was in plan mode.