Plan mode UI stays active after ExitPlanMode succeeds

Description

After calling ExitPlanMode, the tool returns "User has approved exiting plan mode" but the UI continues to show plan mode as active. Subsequent messages still include the plan mode system reminder with all the plan mode constraints.

Steps to Reproduce

1. Enter plan mode via /plan
2. Do some work, write a plan file
3. Call ExitPlanMode — get confirmation "User has approved exiting plan mode"
4. UI still shows plan mode active
5. System reminders in subsequent turns still include plan mode instructions

Expected Behavior

After ExitPlanMode returns success, plan mode should be fully deactivated — both in the UI indicator and in system reminders sent to the model.

Observed Behavior

ExitPlanMode returns success but plan mode persists. Had to work around it by ignoring the stale state. Happened multiple times in the same session.

Environment

• Claude Code CLI on macOS (Darwin 25.5.0)
• Model: claude-opus-4-6 (1M context)