hive: versioned central config with layered targeting ...

The hive now stores one config document for the fleet: a list of layers,
each with an optional match (names/instance_ids/tags), appended as immutable
versions. Publish, history, rollback and status are admin-only (admin key or
loopback); the swarm key reads config, so a compromised instance cannot
rewrite what the fleet runs.

Delivery has three paths that degrade into each other: a push notification,
a 'you are behind' hint in the announce reply, and a periodic sweep over
instances that still report an older hash. Acks are tracked per instance and
version, so the status view answers who took the config and who needs a nudge.

A layer list is validated as a whole; an empty list is a legal document that
means 'no overlay'.