Thread last-request and cache usage fields through the exec event bridge (N3 WP6)

Why: squad consumes exec.ProcessEvent, which lacked the split usage
fields, so the WP6 accounting could not reach squad runs.
Evidence: GOWORK=off go vet green; additive fields on ProcessEvent.
Impact: additive only.
Refs: Agent-Native#35 #37, N3 plan WP6

exec/backend.go

@@ -57,6 +57,11 @@ type ProcessEvent struct {
 	// PromptTokens / CompletionTokens 在 Type=usage 时填写。
 	PromptTokens     int
 	CompletionTokens int
+	// 以下在 Type=usage 或 message_end 时填写（N3 WP6：上一请求与缓存命中拆分）。
+	LastPromptTokens     int
+	LastCompletionTokens int
+	CacheReadTokens      int
+	CacheWriteTokens     int
 }
 
 const (