[Bug]: Default Merge Action not respected

### Platform

macOS

### Operating system version

macOS Tahoe 26.3.1 (a)

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

0.17.0

### Bug description

The default merge action is no longer respected. It always goes to merge locally, even though I have the default merge action set to create a pull request.

### Steps to reproduce

In Settings, under Behavior, set the default merge action to "Create Pull Request." Then, in a workspace, observe that the button says "Merge" and not "Create Pull Request".

### Relevant log output

```shell

```

从 Settings 的 Behavior 开始，跟踪 workspace 显示其 merge 按钮时如何读取默认 merge 操作。将默认值设置为 "Create Pull Request" 进行复现；完成的标准是按钮显示 "Create Pull Request" 而不是 "Merge"，并且设置仍然得到遵守。

由索引模型根据 Issue 内容生成。