`.github` folder in a project is not used in the worktree

### Feature summary

Use the `.github` (and private) folders for projects as well

### What problem are you trying to solve?

The session does not take the `.github` or `github-private` folders into account when creating a worktree of a mutli repo project. I created a helpful canvas in a session which I can find in the `.github` repo, yet when I create a new session I cannot use the canvas.

### Proposed solution

Also use the `.github` or `github-private` folders in the worktree.

### Workflow impact

Especially used for multi-repo projects, misses config and functionality right now, including org level Copilot instructions.

### Installation context

_No response_

### Additional context

Local folder setup:

Copilot App:

Worktree:

首先跟踪多仓库会话如何创建其 worktree 以及选择仓库文件夹；检查 .github 和 github-private 的处理方式。完成的标准是将这些文件夹包含在 worktree 中，以便画布、配置和组织级 Copilot 指令在新会话中可用。

由索引模型根据 Issue 内容生成。