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.

首先审查 GitHub Copilot App 当前的 worktree 工作流，以及它如何区分 Git 项目和普通文件夹。在提出视觉流程之前，梳理现有的任务工作区和 merge 交互。Done 应包括清晰的工作区说明、审查、保留、merge 和丢弃操作、合并前的比较，以及在隔离工作和主项目中工作之间提供易于访问的选择。

由索引模型根据 Issue 内容生成。