feat: route nofxos data API calls through claw402 x402 payment

When CLAW402_WALLET_KEY env var is set, all nofxos.ai data API calls
(AI500, OI rankings, NetFlow, price rankings) are automatically routed
through claw402.ai with x402 USDC micropayment.

- provider/nofxos/claw402.go: x402 GET request client for data APIs
- provider/nofxos/client.go: claw402 mode support in doRequest()
- kernel/engine.go: auto-detect CLAW402_WALLET_KEY and enable routing
- mcp/payment/x402.go: MakeClaw402SignFunc helper

Without CLAW402_WALLET_KEY, falls back to direct nofxos.ai (backward compat).

kernel/engine.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
 	"nofx/logger"
 	"nofx/market"
 	"nofx/provider/hyperliquid"
@@ -194,6 +195,21 @@ func NewStrategyEngine(config *store.StrategyConfig) *StrategyEngine {
 	}
 	client := nofxos.NewClient(nofxos.DefaultBaseURL, apiKey)
 
+	// If claw402 wallet key is available, route nofxos requests through claw402
+	if walletKey := os.Getenv("CLAW402_WALLET_KEY"); walletKey != "" {
+		claw402URL := os.Getenv("CLAW402_URL")
+		if claw402URL == "" {
+			claw402URL = "https://claw402.ai"
+		}
+		claw402Client, err := nofxos.NewClaw402DataClient(claw402URL, walletKey, nil)
+		if err == nil {
+			client.SetClaw402(claw402Client)
+			logger.Infof("🔗 NofxOS data routed through claw402 (%s)", claw402URL)
+		} else {
+			logger.Warnf("⚠️ Failed to init claw402 data client: %v (using direct nofxos.ai)", err)
+		}
+	}
+
 	return &StrategyEngine{
 		config:       config,
 		nofxosClient: client,