Cover: the four scope types (account-wide, one cost source, one team, one
tag value — team is really a special case of tag-scoped), ok/warning/exceeded
states, alert channels (Slack/Teams/webhook/email) and subscriptions,
scheduled digest reports as a distinct concept from budget alerts, and that
budgets always evaluate on billed cost (see Billed vs. amortized) regardless
of what the chart next to them is displaying.
