Fork Conversation always creates a new worktree

Forking conversation is very useful to try out different ideas, but these don't always include code changes and having to wait minutes for a new worktree creation is very annoying, and it is hard to merge the changes back to the original branch.

Please let me choose, I should be able to just for the conversation on the same directory

首先追踪 Fork Conversation 流程以及它创建新 worktree 的位置。确定用户如何在当前目录和新 worktree 之间进行选择；完成标准是会话分叉可以使用所选位置，而不会强制创建不必要的 worktree。

由索引模型根据 Issue 内容生成。