[General] When changing storage location move existing repos, worktrees and session state

On Windows by default the Copilot App stores everything under the user profile. Once you've started doing things and realise that this is happening, and want to move everything to the dev drive you realise changing the setting only works with new sessions and repos.

When changing storage location the app should move existing repos, worktrees and session state to the chosen location, as presumably the user choses the location because it has more space, or is faster, or whatever.

---
| Field | Value |
| --- | --- |
| App version | 1.0.15 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 5 |

未指定文件、测试或入口点。首先跟踪存储位置设置，以及现有仓库、worktree 和会话状态是如何被定位的。完成的标准是：更改设置会将这些现有项目移动到所选位置，而不仅仅是移动新会话创建的数据。

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