Import WhatsApp exports before live messages

"onechat import [-n] ZIP..." reads the chat exports of the phone, makes
one marked WhatsApp batch for each chat and day, and rebuilds the memory
with them in date order. It takes only messages from before the first
live batch, maps the owner's profile name to "You", and skips the chat
days that an earlier import added. With -n it prints the plan and the
estimated calls and makes no model call.

The names, the chat IDs, and the owner come from the WhatsApp session
through the library's store, so the import runs only while the daemon is
stopped and needs no network.

Issue: b0b8bf9