fix(server): preserve unrelated branch upstreams when opening fork PRs (#18413)

Co-authored-by: SAPHID <saphid@users.noreply.github.com>
Co-authored-by: GPT-6.1 Sol <noreply@openai.com>
3cc98ba567Alex Southwell committed on 10/18/2026, 8:09:41 PM· committed by GitHubparentdfda928
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