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.

從 `gh stack init` 入口開始，追蹤當 `$GIT_DIR` 指向 `.git/worktrees//` 時本機 stack 成員關係的儲存方式。在主要 worktree 和連結的 worktree 之間重現此行為，包括 `worktree remove`；從另一個 worktree 開啟該分支時，stack 成員關係仍然可用，即表示完成。

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