[Bug]: Reverting loses context

### Platform

macOS

### Operating system version

Tahoe 26.3.1 (a)

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

0.19.0

### Bug description

When using the "Revert to here" feature, it seems that the context gets lost.

### Steps to reproduce

Ask it something:

Then revert and re-ask it the same thing:

### Relevant log output

```shell

```

The issue names no source files, tests, or entry points. Start by reproducing “Revert to here” on macOS with the reported conversation, then trace the revert flow and compare the follow-up context with the original. Done means the same question retains the expected context after reverting.

索引モデルが issue の本文から書いたものです。