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.

Start with the existing /fork behavior in Copilot CLI and trace how a fork continues in the CLI or app. Define the choice between creating a worktree and reusing the current checkout, then verify that the fork preserves session context and supports nested-session workflows.

索引モデルが issue の本文から書いたものです。