[Bug]: Merge Fails when Limits Reach

### Platform

macOS

### Operating system version

MacOS Sonoma 14.4.1

### System architecture

Intel (x86)

### PolyScope Version

1.0

### Bug description

I have both Claude Code as well as Codex installed. When I went to merge it states my Claude code exited and that I hit limits, but I’m using the codex model.

When this happens it says the merge was successful but it’s not. If you remove the agent you will lose the work.

### Steps to reproduce

Have both Claude code and codex installed. Use up all your Claude code usage. Switch to codex models then attemp to merge.

### Relevant log output

```shell

```

Reproduce on macOS Sonoma 14.4.1 with both Claude Code and Codex installed: exhaust Claude Code usage, switch to a Codex model, and attempt a merge. Start by tracing the merge flow and its handling of agent exit or usage-limit errors. Done means the merge result is accurately reported and work is not lost when the merge fails.

索引モデルが issue の本文から書いたものです。