A conflict low in a stack blocks merging PRs above it, even when they merge cleanly

**The problem**

Say `main ← PR1 ← PR2`. PR1 has merge conflicts against `main` and can't be merged. PR2 against PR1 is completely fine — no conflicts at all.

Merging is all-or-nothing for the whole chain up to `main`: if any PR in the stack can't be merged, none of them are. So even though PR2's changes are ready and conflict-free relative to PR1, there's no way to merge it — it's stuck behind PR1's unrelated conflict with `main`.

**Impact**

A single conflict low in a stack blocks everything built on top of it, even work that's fully ready and has nothing to do with that conflict. Teams end up stuck waiting on someone to resolve the bottom PR's conflict before any of the PRs above it can move forward, with no way to make progress on the parts that are actually ready.

This also blocks deploying feature branches to a staging environment for testing — a common practice at many companies. In this scenario, the feature branch is PR1's branch (the one with the conflict against `main`): deploying it to staging means merging it, and that merge is exactly what's blocked, so the branch can't be deployed until the unrelated conflict is resolved.

Beginne damit nachzuverfolgen, wie gh-stack feststellt, ob gestapelte Pull Requests gemergt werden können und wie es Branches behandelt, die für ein Staging-Deployment vorgesehen sind. Lege das gewünschte Verhalten für einen konfliktfreien Pull Request oberhalb eines konflikthaften fest, einschließlich der Frage, wie seine Merge-Bereitschaft und sein Deployment-Pfad dargestellt werden sollen. Als abgeschlossen gilt die Arbeit, wenn höherliegende Pull Requests unabhängig fortschreiten können, ohne ihre Abhängigkeiten fälschlicherweise zu umgehen.

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