[Bug]: Pushing commit to a fork's PR goes to the wrong remote

### Platform

macOS

### Operating system version

macOS Tahoe 26.3.1 (a)

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

0.15.0

### Bug description

If I create a workspace from a PR, and that PR belongs to an outside contributor (i.e. its from a fork), when I push a commit Polyscope/Claude will push it to my own repo's origin (with the same branch name used in the PR) rather than back to the fork's branch.

### Steps to reproduce

- Create workspace from pull request. The PR must be from a fork.
- Make a change
- Click the commit button

### Relevant log output

```shell

```

首先，使用來自 fork 的 pull request，重現從建立 workspace 到 commit 按鈕的流程。追蹤 push 期間如何選取 PR 分支和遠端儲存庫，然後確認 commit 會傳回 fork 的分支，而不是使用者的 origin。

由索引模型根據 Issue 內容生成。