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.

首先追蹤 gh-stack 如何判定堆疊的 pull request 是否可以合併，以及它如何處理用於 staging 部署的分支。定義位於有衝突的 pull request 之上的無衝突 pull request 的預期行為，包括應如何表示其可合併狀態和部署路徑。完成的標準是，較上層的 pull request 可以獨立推進，而不會錯誤地繞過其相依項目。

由索引模型根據 Issue 內容生成。