Github stack does not persist across worktrees

When opening a branch in a worktree and running `gh stack init`, it does not keep track of the stack locally. `gh-stack` keeps stack membership in local state tied to the checkout where `init` ran. If I create a worktree & use `gh-stack`, the stack doesn't persist on my machine, even if I open the branch back in my main worktree. Inside a worktree, $GIT_DIR is .git/worktrees// — so `worktree remove` deletes the stack state with it

This is confusing & not obvious. Additionally, a modern LLM workflow may involve different sub-agents in different worktrees. This makes gh-stack worse than, say git-spice for this purpose, since I can't easily update my work if I checkout the branch on my main worktree.

The stack _is_ stored remotely on Github, but THAT doesn't always work - seems like GitHub only records a stack for a chain of two or more PRs.

从 `gh stack init` 入口开始，跟踪当 `$GIT_DIR` 指向 `.git/worktrees//` 时本地 stack 成员关系的存储方式。在主 worktree 和关联 worktree 之间复现该行为，包括 `worktree remove`；当从另一个 worktree 打开该分支时，stack 成员关系仍然可用，即视为完成。

由索引模型根据 Issue 内容生成。