Active Configuration edit section ignored in --replay --edit

[rxgrant]

As a user, when I change the model name in a --replay --edit, such as when trying to downgrade the model after a refusal, I expect the edited configuration to be parsed and applied.

# Active Configuration

> NOTE: You can edit this configuration to apply it to the current conversation.
>       If the configuration is invalid, the editor will re-open.
>
>       Only the most relevant configuration is shown here, but you can add any
>       configuration properties you want to apply.

```toml
[assistant.model.id]
provider = "anthropic"
name = "claude-opus-4-8"

[assistant.model.parameters]
reasoning = "auto"
```

Analysis: this is a delayed-application error that you could call an off-by-one error. The state is stored but the previous state is applied. A second --replay --edit with no edits will apply the model change.

[JeanMertz]

Correct, the solution to this is a bit more involved, because I want to avoid making AppConfig mutable after the initial load pipeline, to keep the mental model of our configuration pipeline easier to manage.

I have an RFD up that proposes a solution, but I need to update it first to the latest state of the code, since the editor-invocation path has gotten some significant changes in recent weeks.