New-session branch picker only enumerates the 'origin' remote; repos with other remote names show only the default branch

### Summary

When creating a new session for a multi-repo collection project, the branch picker only enumerates branches for repos whose remote is named `origin`. Repos that use a differently-named remote (e.g. `github`) fall back to showing only the default branch (`main`), and their `repo_full_name` is reported as `unknown`.

### Repro

Collection project with two member repos:

- `frontend` — remote named `origin` -> all branches are listed in the new-session branch picker.
- `backend` — remote named `github` (same GitHub URL, just a different remote name) -> only `main` is selectable, and the repo shows up as `repo_full_name: unknown`.

Both remotes point at valid GitHub repos:

```
origin https://github.com/Org/frontend.git
github https://github.com/Org/backend.git
```

### Expected

Branch enumeration and repo resolution should work regardless of the remote name. The app should inspect all configured remotes (or at least the one backing the tracked/default branch) rather than hard-coding a lookup for `origin`.

### Actual

Only the `origin` remote is consulted. Repos without an `origin` remote are limited to the default branch and are labeled `unknown`.

### Workaround

Add or rename the remote to `origin`:

```
git remote rename github origin
```

After this, the new-session branch picker lists all branches for that repo.

从新会话分支选择器开始，追踪它如何发现远程仓库并解析 repo_full_name。使用远程仓库名为 github 的仓库进行复现，然后验证所有分支都会被列出，并且无需重命名远程仓库，该仓库不再显示为未知。

由索引模型根据 Issue 内容生成。