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.

Empieza por rastrear cómo gh-stack determina si los pull requests apilados se pueden fusionar y cómo gestiona las ramas destinadas al despliegue en staging. Define el comportamiento deseado para un pull request sin conflictos situado por encima de otro con conflictos, incluido cómo deben representarse su disponibilidad para fusionarse y su ruta de despliegue. Se considera terminado cuando los pull requests superiores pueden avanzar de forma independiente sin omitir incorrectamente sus dependencias.

Escrito por el modelo de indexación a partir del texto del issue.