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.

Inizia analizzando come gh-stack determina se le pull request impilate possono essere unite e come gestisce i branch destinati al deployment in staging. Definisci il comportamento desiderato per una pull request senza conflitti posta sopra una in conflitto, incluso come debbano essere rappresentati il suo stato di idoneità al merge e il suo percorso di deployment. Il lavoro è completato quando le pull request superiori possono avanzare indipendentemente senza aggirare erroneamente le proprie dipendenze.

Scritto dal modello di indicizzazione a partire dal testo della issue.