How do I set the email where agent reports are sent?

The agent report email functionality is configured in body executor control https://github.com/cfengine/masterfiles/blob/3.18/controls/cf_execd.cf. It defaults to root@$(def.domain) which is configured in bundle common def https://github.com/cfengine/masterfiles/blob/3.18/def.cf.

See also: def.mailto.

How do I disable agent email output?

You can simply remove or comment out the settings.

The Masterfiles Policy Framework will disable agent email when the class cfengine_internal_disable_agent_email available in controls/def.cf to switch on/off agent email.