Exiting plan mode not reliable with new GPT-5.6 models

### Describe the bug

When creating a plan with the new GPT-5.6 models (doesn't matter which one), often the interaction ends like this:

`Plan saved to plan.md, with implementation tasks tracked in the session database.`

And then nothing happens, what is expected is that the user is prompted with how to execute this plan, but this almost never happens. What works is to do prompt something like:

`Present me the plan, and ask me to implement with autopilot`

And after that implementation works as expected.

Other models (GPT-5.5, Anthropic, etc) do not have this issue.

### Affected version

`1.0.71`

### Steps to reproduce the behavior

See above

### Expected behavior

See above

### Additional context

N/A

未指定文件、测试或入口点。使用 GPT-5.6 模型在版本 1.0.71 中重现该故障，将其与 GPT-5.5 或 Anthropic 进行比较，并跟踪退出 plan-mode 的过程；完成的标准是保存计划后，预期的实现提示能够可靠地出现。

由索引模型根据 Issue 内容生成。