[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 |

No files, tests, or entry points are named. Start by tracing the storage-location setting and how existing repositories, worktrees, and session state are located. Done means changing the setting moves those existing items to the chosen location, not only data created by new sessions.

索引モデルが issue の本文から書いたものです。