[Bugs] If no `user.email` set an incorrect default `<local user>@github.com` is used instead

**Mood:** 😔
**Category:** Bugs

If no `user.email` is set on a repo (or globally), GH Copilot App appears to have defaulted to `@github.com`. This caused confusion in that my (private) repo began showing GH users as contributors to the project, even though I am the only owner/collaborator. It turns out that my local machine user account name happens to match a (different) GH user.

Granted, setting a Git user name/email should be a first step when cloning a repo, but it's an easy step to forget when setting up a new machine. I feel like GH Copilot App could warn before using a default email.

---
| Field | Value |
| --- | --- |
| App version | 0.2.27 |
| OS | macOS 26.5.1 |
| Theme | Chai |
| Path | /chat |
| Tenure | Week 6 |

Start from the /chat flow and inspect how the app obtains Git user.email when the repository and global configuration are unset. Reproduce the fallback with no configured email, then verify that the app warns before using @github.com rather than silently attributing commits to that address.

索引モデルが issue の本文から書いたものです。