Directory sync: journal keeps more state and can report a 'detached' stop

What

Directory sync keeps a journal, a small record of its state, shared between your machine and the cloud worker that runs a cloud session. This release extends it:

• The worker's journal note now carries linksStopped and an optional basisBranch. Both are restored from the note when the worker starts up again.
• A new decodeWorkerDetached decoder reads a journal whose halted reason is "detached" and returns its detachCause.
• The sync pull step uses it to end sync with a 'detached' reason, but only when the session's peerKeepsFiles option is true. The option defaults to false, and its one caller sets it based on another check.

Why

Sync state survives a worker restart more completely. Where peerKeepsFiles applies, a sync the cloud side has detached from now stops with a clear reason instead of carrying on failing.