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) |

Inizia tracciando il flusso di creazione di GitHub Copilot App worktree session e l’attuale generazione dei nomi dei branch, quindi verifica dove sono definite le impostazioni App globali e specifiche per progetto. Il lavoro è completato quando gli utenti possono configurare un prefisso del branch e i worktree branches appena creati applicano in modo coerente il formato selezionato.

Scritto dal modello di indicizzazione a partire dal testo della issue.