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) |

まず GitHub Copilot App の worktree session 作成フローと現在のブランチ名生成を追跡し、次にグローバルおよびプロジェクト単位の App settings がどこで定義されているかを調査します。ユーザーがブランチのプレフィックスを設定でき、新しく作成された worktree branches に選択した形式が一貫して適用されれば完了です。

索引モデルが issue の本文から書いたものです。