Allow forking a session without creating a worktree

## Summary

As a user, I want to fork a full session while reusing the current checkout so that I can preserve the session context without creating a worktree.

## Problem

Worktrees are not practical for some large repositories because they can take a long time to create or cannot build independently.

## Current workaround

Use `/fork` in Copilot CLI, then continue the forked session in either the CLI or the app.

## Proposed behavior

When forking a session, allow the user to choose between:

- Creating a separate worktree
- Reusing the current checkout

The fork should preserve the parent session's context and support nested-session workflows.

## Expected outcome

Users can work on related tasks from the same session context without creating an impractical worktree.

从 Copilot CLI 中现有的 /fork 行为开始，跟踪 fork 如何在 CLI 或 app 中继续。定义创建 worktree 与复用当前 checkout 之间的选择，然后验证 fork 是否保留会话上下文并支持嵌套会话工作流。

由索引模型根据 Issue 内容生成。