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.

Commencez par suivre le flux New session pour une collection ainsi que le résolveur de branches affiché dans les logs, puis reproduisez le problème avec un dossier parent contenant trois repositories enfants directs et aucun répertoire .git parent. C’est terminé lorsque les repositories enfants sont listés avec des choix de branche et de checkout pour chaque repository, et que les commandes Git branch, worktree, ref et fetch s’exécutent sur chaque enfant plutôt que sur le parent.

Rédigé par le modèle d'indexation à partir du texte de l'issue.