Collection new-session flow treats parent folder as a Git repository

## Affected version
GitHub Copilot app 1.1.14 on macOS 26.6.2.

## What happened?
A parent folder with no `.git` directory contains three direct child Git repositories, all on `main`. After adding the parent through **Projects -> + -> Open folder**, the app correctly registers it as a collection.

Opening **New session** for the collection shows generic single-repository controls (Project: collection name, Workspace: New worktree, Branch: main). The child repositories are not listed, so their branches and checkout modes cannot be selected.

Logs show the branch resolver targeting the non-Git parent:

```
Failed to get current branch path=~/gitrepos/spec-forge-all
error=git ["symbolic-ref", "HEAD"] failed:
fatal: not a git repository (or any of the parent directories): .git
```

The same failure occurs for `git worktree list`, `for-each-ref`, and `git fetch` against the parent.

## Steps to reproduce
1. Create a parent folder containing multiple direct child Git repositories, with no `.git` at the parent.
2. Add the parent using **Projects -> + -> Open folder**.
3. Click **New session** for the collection.
4. Observe one branch selector for the parent and no child-repository list.

## Expected behavior
Enumerate collection members and allow branch/checkout selection for each child. Run Git branch commands against each child, not the parent folder.

## Additional context
**Local repository** is a partial workaround but does not restore isolated multi-repository worktrees or per-repository branch selection. Related but narrower: #786 covers per-repository default branches.

首先跟踪 collection 的 New session 流程以及日志中显示的 branch resolver，然后使用一个包含三个直接子 repository 且没有父级 .git 目录的父文件夹进行复现。完成标准是：子 repository 会以每个 repository 各自的 branch 和 checkout 选项列出，并且 Git branch、worktree、ref 和 fetch 命令会针对每个子 repository 执行，而不是针对父 repository 执行。

由索引模型根据 Issue 内容生成。