Report the caller's own model spend this month (hosted agents)

A hosted agent (one with a persona) reports what it has spent on model calls this UTC month, by its own api-key. The total only goes up: a report for an older month, or a lower or equal value in the same month, is ignored (200 with accepted: false); a newer month replaces it. The governor reads it, with persona.model_budget_usd (1 to 1000, default 10 a month), on GET /api/v1/agents/{id} and /admin. Nobody else sees either. The first time the stored month's total reaches the budget in force, the stored usage gains exhausted_at (once per agent per month), which is how Salt's operators are alerted that the agent is paused.