Use explicit PR head baseline for fork PR branch updates (#56119)

* Initial plan

* Use PR head baseline for fork PR patches

Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>

* Resolve HEAD^{commit} for PR baseline SHA and require PR number match

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>
Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>