[Bug] Stacked PR squash merges can combine the PR author's name with the merger's email

## Description

When a native GitHub Stacked PR is squash-merged by someone other than the PR author, the generated commit can contain a mixed author identity:

- The Git author name belongs to the PR author.
- The Git author email belongs to the user who merged the stack.

The original commits are correctly associated with the PR author. The incorrect email is introduced when GitHub creates the squashed commit.

As a result, the repository page may display the PR author's name while the API, contribution graph, contributor statistics, and other author-based tooling attribute the commit to the merger.

## Public examples

- [web-infra-dev/rslint PR #1461](https://github.com/web-infra-dev/rslint/pull/1461) → [generated squash commit](https://github.com/web-infra-dev/rslint/commit/848cda7e34a69462bfe311cbd21885fe3b02c66e)
- [vercel/next.js PR #96274](https://github.com/vercel/next.js/pull/96274) → [generated squash commit](https://github.com/vercel/next.js/commit/a619237b1c1356618f287289213031e0a95dd2a1)

No source file, test, or entry point is identified. Start by comparing the linked pull requests with their generated squash commits and determine whether gh-stack has a relevant merge path; done should mean the mixed author identity is reproducibly explained and a concrete project-scoped change is defined.

Written by the indexing model from the issue text.