Stop hooks fail with ENOENT after worktree cleanup

Stop hooks fail with ENOENT: no such file or directory, posix_spawn '/bin/sh' when the session's cwd is a worktree that was removed (post-merge cleanup). The OS can't resolve the deleted cwd to
spawn the hook process. Node.js misleadingly reports ENOENT on /bin/sh rather than the cwd. Suggested fix: fall back to the original repo root or $HOME when cwd doesn't exist.