feat: add Claw402 (claw402.ai) x402 USDC payment provider

Add Claw402Client for claw402.ai's x402 micropayment gateway (Base USDC).
Supports 15+ AI models (GPT-5.4, Claude Opus, DeepSeek, Qwen, Grok, etc.)
with per-model endpoint routing.

- mcp/claw402.go: new client with model→endpoint mapping, x402 v2 payment flow
- mcp/blockrun_base.go: extract shared signX402Payment() for reuse
- Register "claw402" provider in all 6 consumer switch statements:
  api/server.go, api/strategy.go, trader/auto_trader.go,
  telegram/bot.go, debate/engine.go, backtest/ai_client.go

debate/engine.go

@@ -103,6 +103,8 @@ func (e *DebateEngine) InitializeClients(participants []*store.DebateParticipant
 			client = mcp.NewBlockRunBaseClient()
 		case "blockrun-sol":
 			client = mcp.NewBlockRunSolClient()
+		case "claw402":
+			client = mcp.NewClaw402Client()
 		default:
 			client = mcp.New()
 		}