`hive-<hive>` is each hive's read-only forge account, and an agent's forge
account takes the agent's name, so an agent called `hive-<x>` would claim
hive `<x>`'s account. `hive_types::reserved_agent_prefix_violation` names
the rule, and `broken_name_rules` applies it, so both creation paths that
go through it (`POST /api/agents` and placing an undeclared agent on a
hive) refuse such a name for a new agent. An agent already in the roster
keeps its name with a warning, as for every other naming rule.
The email check between hive and agent accounts stays as the second
guard, for agents created before this rule.
Refs #4851
4e7142ae53