Cloud sessions: inactive persistentVolume option, and a directory-sync note for every worktree

What

Two changes touch how cloud sessions are started, for example with --remote:

• persistentVolume is a new option that now runs from the command-line option parser through cloud and pool session creation into the session-create request. It cannot be turned on. It starts as false, nothing sets it otherwise, and no command-line flag, environment variable or setting feeds it. Even a true value would add nothing to the request, because the helper that should write it (E_r) returns nothing in this build.
• appliesToEveryWorktree is a new setting for the directory-sync consent dialog, the prompt asking whether your local files may be synced to a cloud session. A worktree is an extra checkout of the same git repository in another folder. When you are in a worktree whose root differs from the repository's main root, the dialog shows a dim note under the repository name: "Your answer applies to every worktree of this repository."

Why

persistentVolume has no effect you can see or use yet. The worktree note matters in practice: allowing or refusing sync from one worktree also decides it for the repository's other worktrees.