Plan mode not re-activated after ExitPlanMode for subsequent tasks

Bug Description

After calling ExitPlanMode and completing a task implementation, the plan mode system instruction (Plan mode is active) is not sent for subsequent user messages in the same session - even though the UI status bar continues to show "plan mode on".

This causes the assistant to proceed with direct implementation instead of planning for follow-up tasks.

Steps to Reproduce

1. Start a session with plan mode enabled (status bar shows plan mode on)
2. User sends a task request
3. Assistant receives Plan mode is active system reminder - works correctly
4. Assistant writes plan, calls ExitPlanMode
5. User approves plan, assistant implements the task
6. User sends a NEW task request (different from the first)
7. Bug: Assistant does NOT receive the Plan mode is active system reminder
8. Assistant proceeds with direct implementation, bypassing plan mode
9. Status bar still shows plan mode on - misleading the user

Expected Behavior

After ExitPlanMode completes and the task is implemented, subsequent user messages should again include the Plan mode is active system instruction, matching what the UI displays.

Actual Behavior

The Plan mode is active system reminder is only sent for the first task. After ExitPlanMode, it is not re-injected for follow-up tasks, causing a mismatch between:

• UI status bar: shows plan mode on
• System instructions to assistant: no plan mode reminder present

Workaround

User can toggle plan mode off and on again with Shift+Tab to re-sync the state.

Environment

• Claude Code CLI
• Model: Opus 4.6 (1M context)
• OS: macOS (Darwin 25.3.0)