Allow customization of worktree branch name prefixes

Allow customization of worktree branch name prefixes

When GitHub Copilot App creates a worktree session, it automatically creates a branch such as:

nishijima267-fantastic-goggles

I would like an option to configure the branch naming format, for example:

copilot/nishijima267/fantastic-goggles

Using a "copilot//" prefix would make it immediately clear that the branch was created by Copilot, identify its owner in shared repositories, and group Copilot branches consistently.

Ideally, this could be configured globally or per project in the App settings.

Repository instructions such as AGENTS.md or .github/copilot-instructions.md cannot solve this because the branch and worktree are created before those instructions are read.

---
| Field | Value |
| --- | --- |
| App version | 1.1.12 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 2 (Week 1) |

首先跟踪 GitHub Copilot App worktree session 的创建流程以及当前的分支名称生成逻辑，然后检查全局和按项目划分的 App settings 定义位置。完成的标准是用户可以配置分支前缀，并且新创建的 worktree branches 一致地应用所选格式。

由索引模型根据 Issue 内容生成。