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.

Beginne beim Einstiegspunkt `gh stack init` und verfolge, wie die lokale Stack-Zugehörigkeit gespeichert wird, wenn `$GIT_DIR` auf `.git/worktrees//` zeigt. Reproduziere das Verhalten über einen Hauptarbeitsbaum und einen verknüpften Arbeitsbaum hinweg, einschließlich `worktree remove`; fertig ist es, wenn die Stack-Zugehörigkeit verfügbar bleibt, wenn der Branch aus einem anderen Arbeitsbaum geöffnet wird.

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