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.

未指定檔案或測試。首先定位 Copilot CLI 的工作階段建立分支選擇器，並追蹤其分支資料的載入方式。更新選擇器，讓僅存在於本機的分支和遠端分支都可用，然後執行相關的 CLI 測試，或手動驗證工作階段可以基於尚未推送的本機分支建立。

由索引模型根據 Issue 內容生成。