Import WhatsApp history into a child

The import command selects an existing direct child with --agent NAME
and uses its log, prompt, tools, cache key, and usage records. The root
stays the default target and supplies live cutoff history for a child.
Plans name the target without model calls or changes to its files.

Child rebuilds keep their staging directories and timestamped backups
outside children. Only complete child configs appear in that directory,
so a backup or interrupted rebuild cannot stop daemon startup.