Carry cache tokens on message_end events (N3 WP6 follow-up)

Why: hosts accumulating per message_end could not see cache hits; the
fields now ride the same event as the per-request totals.
Evidence: GOWORK=off go test green.
Impact: additive event fields.
Refs: Agent-Native#35

runtime/loop.go

@@ -352,7 +352,7 @@ func Run(ctx context.Context, cfg Config, user llm.Message) (Result, error) {
 			return Result{}, err
 		}
 		messages = append(messages, last)
-		emit(cfg, Event{Type: "message_end", Message: last, PromptTokens: end.Usage.PromptTokens, CompletionTokens: end.Usage.CompletionTokens, TotalTokens: end.Usage.TotalTokens})
+		emit(cfg, Event{Type: "message_end", Message: last, PromptTokens: end.Usage.PromptTokens, CompletionTokens: end.Usage.CompletionTokens, TotalTokens: end.Usage.TotalTokens, CacheReadTokens: end.Usage.CacheReadTokens, CacheWriteTokens: end.Usage.CacheWriteTokens})
 
 		if last.StopReason == llm.StopToolCalls && len(last.ToolCalls) > 0 {
 			if reason := budgetExceeded(cfg.Budget, usage, toolCalls, time.Now()); reason != "" {