Data-driven strategy template: Gather entities → Broadcast events.
Fan-out pattern. Calls a gatherer that returns a list of entities,
then broadcasts a bus event for each entity to trigger downstream actors.
All gathering and broadcasting happens in init — no step loop needed.
Strategy config shape
%{
"gatherer" => "active_projects",
"event_type" => "project_health.check_requested",
"entity_id_field" => "id",
"entity_payload_fields" => ["id", "name"]
}The gatherer must return {:ok, %{entities: [%{...}, ...]}} where
each entity is a map. The entity_id_field and entity_payload_fields
control what gets broadcast.