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.

首先跟踪 gh-stack 如何确定堆叠的 pull request 是否可以合并，以及它如何处理用于 staging 部署的分支。定义位于存在冲突的 pull request 之上的无冲突 pull request 的预期行为，包括应如何表示其可合并状态和部署路径。完成的标准是，更高层的 pull request 可以独立推进，而不会错误地绕过其依赖项。

由索引模型根据 Issue 内容生成。