Support stacks across Git worktrees

[skarim]

Makes gh stack work with existing Git worktrees, whether they hold independent stacks or different layers of the same stack.

• Share stack tracking across worktrees so moving a branch or removing a worktree no longer loses its stack. Migrate compatible existing state with backups; conflicting definitions require resolution rather than guessing.
• Run rebase and sync in each affected branch's worktree, with dirty/busy checks, coordinated mutations, and conflict recovery that can be continued or aborted from another worktree.
• Adopt branches already checked out elsewhere without moving them. Add --print-path for shell-assisted navigation, and preserve occupied branches during pruning.

Requires Git 2.36+. This adds compatibility with existing worktrees, not worktree creation/removal or automatic stashing. Distributed modify support follows in #521.

Related issues

• Closes Github stack does not persist across worktrees #459
• Closes gh stack rebase silently succeeds when git rebase --onto fails to start (in branch checked out in another worktree) #35
• Closes sync might not work with worktree #87