Always call plan_exit when finished planning in plan mode

The user has repeatedly established as a standing instruction (across June 16, July 21, and July 28 sessions) that the assistant must invoke the plan_exit tool at the very end of its turn once the plan file is written and ready for approval, to signal planning is complete. This applies whenever plan mode is active: the turn should only end via the question tool (to clarify ambiguities) or plan_exit. Never end the turn with a textual summary implying completion without actually calling plan_exit. This directive overrides any prior assumption that a written plan file or 'done' message suffices.