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.

Inizia dal punto di ingresso `gh stack init` e traccia come viene memorizzata l’appartenenza locale allo stack quando `$GIT_DIR` punta a `.git/worktrees//`. Riproduci il comportamento tra un worktree principale e un worktree collegato, incluso `worktree remove`; il lavoro è completato quando l’appartenenza allo stack rimane disponibile aprendo il branch da un altro worktree.

Scritto dal modello di indicizzazione a partire dal testo della issue.