Multi-store-sync writes gain a 'publish fence' that can drop writes and now return an explicit publish outcome

When Claude Code syncs data across multiple storage backends, the write function now checks an optional publishFence on the backend, both before starting and again right before the final write. If the fence's file no longer exists, or its mayPublish() check returns false, the write is dropped and the old content is re-fetched instead of being overwritten.

The function also now returns an explicit "published" or "dropped" result instead of returning nothing.