feat: add channel dimension to GA4 AI usage tracking

Distinguish claw402, blockrun, and native direct API calls in telemetry.

config/config.go

@@ -129,6 +129,7 @@ func Init() {
 		telemetry.TrackAIUsage(telemetry.AIUsageEvent{
 			ModelProvider: usage.Provider,
 			ModelName:     usage.Model,
+			Channel:       usage.Channel(),
 			InputTokens:   usage.PromptTokens,
 			OutputTokens:  usage.CompletionTokens,
 		})