Make Copilot App worktree management more visual and beginner-friendly

The current worktree workflow in GitHub Copilot App feels too complex for non-technical users.
When Copilot creates a separate worktree for a task, users may not understand:

- why a new folder and branch were created;
- where the changed files are located;
- whether their main project has been changed;
- how to review, keep, merge, or discard the work;
- why they need to use Git commands in PowerShell or CMD to complete a simple task.

The distinction between Git projects and regular folders is also not clear enough. The different project icons provide a hint, but the app does not explain the practical consequence: only Git projects create isolated worktrees for coding sessions.
Please consider a workflow similar to Codex, with a clearer visual interface for task changes and worktree management:

1. Show a plain-language explanation when a worktree is created.
2. Clearly label the main project and each task workspace.
3. Provide visible buttons for **Review changes**, **Keep changes**, **Merge into main project**, and **Discard task**.
4. Show a simple comparison view before merging.
5. Handle common merge steps in the UI instead of requiring terminal commands.
6. Let users choose whether to use an isolated workspace or work directly in the main project, with clear safety warnings.
7. Provide a guided first-time tutorial for Git and worktrees.

The current isolation model is valuable for safety and parallel work, but it should not require users to understand branches, worktrees, or command-line Git before they can successfully use Copilot App.

Commencez par examiner le workflow actuel de worktree de GitHub Copilot App et la manière dont il distingue les projets Git des dossiers ordinaires. Cartographiez l’espace de travail de tâche existant et les interactions de merge avant de proposer le flux visuel. Done doit inclure des explications claires de l’espace de travail, des actions pour examiner, conserver, fusionner et supprimer, une comparaison avant la fusion, ainsi qu’un choix accessible entre un travail isolé et un travail dans le projet principal.

Rédigé par le modèle d'indexation à partir du texte de l'issue.