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.

Start with the `gh stack init` entry point and trace how local stack membership is stored when `$GIT_DIR` points into `.git/worktrees//`. Reproduce the behavior across a main worktree and a linked worktree, including `worktree remove`; done means stack membership remains available when the branch is opened from another worktree.

索引モデルが issue の本文から書いたものです。