New Worktree checkbox should be remembered per folder

* in certain folders, I always use worktrees.
* in certain folders, I never use worktrees.
* in certain folders, I use worktrees based on the task
* :bug: the current remembering of the toggle in a global state from the previous session I started does not work well for me, especially e.g. with `vscode` vs `vscode-internalbacklog`.
* I think it would be better if this were remembered per folder
* If the user picks a brand new folder, we can use the state of the very last sent session

```
Version: 1.139.0-insider
Commit: 046944034292b5479b4e9a50ad1a508033ffb64f
Date: 2026-09-17T03:41:14Z
Electron: 43.6.0
ElectronBuildId: 15274685
Chromium: 150.0.7871.250
Node.js: 24.20.0
V8: 15.0.245.31-electron.0
@github/copilot: 1.0.84-5
@github/copilot-sdk: 1.0.13
OS: Darwin arm64 25.6.0
```