Revert back to Plan mode after a plan was implmented

### Describe the feature or problem you'd like to solve

It should be possible to have Planning the default mode. But after a plan is implemented in autopilot it stays in autopilot

### Proposed solution

Current workflow:

Plan -> Autopilot -> Complete

Actual result:
The session remains in Autopilot/implementation mode.

Expected result:
After a task is completed, the agent should return to Plan mode
(or provide a user preference to do so).

Rationale:
I use Plan mode as a safety mechanism. Every new task should start
with planning and review before implementation begins. Remaining in
Autopilot increases the likelihood of unintended code changes when
starting the next unrelated task.

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue does not identify files or tests. Start by locating the Plan and Autopilot mode transition used when an autopilot task completes, then verify that completion returns the session to Plan mode or applies the requested user preference.

索引モデルが issue の本文から書いたものです。