Branch picker should show local branches, not only remote branches

## Description

When creating a new session in the Copilot CLI app, the branch picker only lists branches from GitHub (remote). Local-only branches that haven't been pushed are not visible.

## Use case

A common developer workflow is to work on a local branch first and push to remote only when the work is done. Currently this workflow is not supported because:

1. You create a local branch and start working
2. You want to open a Copilot CLI session based on that branch
3. The branch doesn't appear in the picker because it hasn't been pushed yet

## Expected behavior

The branch picker should list both local and remote branches, allowing users to base their session on a local branch without having to push it first.

## Current behavior

Only remote (GitHub) branches are shown in the branch picker.

未指定文件或测试。首先定位 Copilot CLI 的会话创建分支选择器，并跟踪其分支数据的加载方式。更新选择器，使仅存在于本地的分支和远程分支都可用，然后运行相关的 CLI 测试，或手动验证会话可以基于尚未推送的本地分支创建。

由索引模型根据 Issue 内容生成。