feat(strategy): English-only XYZ stock prompt + flat-account aggression + tier promote

The strategy prompt the LLM saw for a Chinese-language single-symbol US
stock trader was an incoherent zh/en patchwork — schema in Chinese,
role definition in English, hard constraints in English, custom prompt
back in Chinese — with crypto-flavored BTC/ETH vs Altcoin labelling that
made no sense for ARM-USDC. The LLM responded by being conservative and
boring. When it finally tried to open, the validator rejected the order
because the validator classified the stock as an altcoin (1x equity cap
= 112 USDT max) while the prompt said 5x cap (= 559 USDT).

- kernel/engine_prompt.go (BuildSystemPrompt): all eight prompt sections
  now respect e.GetLanguage() consistently. For single-symbol
  Hyperliquid XYZ assets (US stocks, commodities, forex) we additionally
  force the language to English regardless of the strategy's stored
  language — US-equity reasoning lands better in English and prevents
  the language-mix incoherence. The Hard Constraints section drops the
  BTC/ETH vs Altcoin two-tier split when the strategy trades a single
  instrument and shows one Position Value Limit line tagged with the
  actual symbol. The JSON example uses that symbol instead of the
  legacy BTCUSDT/ETHUSDT. The legacy stored custom_prompt (which was
  Chinese for stock quick-creates) is replaced for XYZ assets by
  buildXYZStockCustomPrompt — a built-in English long-only stock
  briefing that includes a Flat-Account Rule: when Current Positions
  is None, the agent MUST open a long this cycle (size 40-60% probing
  if technicals are mixed, 80-100% on a confirmed breakout). This is
  the "be in the market, not on the sidelines" stance the quick-trade
  flow needed; wait/hold are reserved for when a position already
  exists.

- kernel/engine_position.go + trader/auto_trader_risk.go + agent/trade.go:
  Hyperliquid XYZ assets now use the BTC/ETH higher tier rather than
  the altcoin tier in all three position-value enforcement points. A
  shared isMajorAsset / isMajorTradeSymbol helper treats BTC/ETH crypto
  perps AND any IsXyzDexAsset symbol as the higher tier. With 5x
  equity cap, the AI's confident-open decisions on US stocks now pass
  validation instead of erroring out with "altcoin single coin position
  value cannot exceed 112 USDT".

Net result: on a flat US-stock single-symbol trader, the agent opens
a sized position with stop-loss and take-profit on the very first
flat cycle, manages it (trail / partial / cut), and reports honestly
to the user. The "agent does nothing" complaint is closed.

trader/auto_trader_risk.go

@@ -3,6 +3,7 @@ package trader
 import (
 	"fmt"
 	"nofx/logger"
+	"nofx/market"
 	"strings"
 	"time"
 )
@@ -183,6 +184,18 @@ func isBTCETH(symbol string) bool {
 	return strings.HasPrefix(symbol, "BTC") || strings.HasPrefix(symbol, "ETH")
 }
 
+// isMajorAsset returns true for assets that should use the BTC/ETH higher
+// position-value tier rather than the altcoin (1x equity) tier. This covers
+// BTC/ETH crypto perps AND Hyperliquid XYZ assets (US equities, commodities,
+// forex) — none of which are "altcoins" and all of which deserve the higher
+// per-position cap so the AI can actually take meaningful positions.
+func isMajorAsset(symbol string) bool {
+	if isBTCETH(symbol) {
+		return true
+	}
+	return market.IsXyzDexAsset(symbol)
+}
+
 // enforcePositionValueRatio checks and enforces position value ratio limits (CODE ENFORCED)
 // Returns the adjusted position size (capped if necessary) and whether the position was capped
 // positionSizeUSD: the original position size in USD
@@ -195,12 +208,14 @@ func (at *AutoTrader) enforcePositionValueRatio(positionSizeUSD float64, equity
 
 	riskControl := at.config.StrategyConfig.RiskControl
 
-	// Get the appropriate position value ratio limit
+	// Get the appropriate position value ratio limit. BTC/ETH AND Hyperliquid
+	// XYZ assets (US stocks etc.) use the higher tier; pure altcoins use the
+	// lower tier.
 	var maxPositionValueRatio float64
-	if isBTCETH(symbol) {
+	if isMajorAsset(symbol) {
 		maxPositionValueRatio = riskControl.BTCETHMaxPositionValueRatio
 		if maxPositionValueRatio <= 0 {
-			maxPositionValueRatio = 5.0 // Default: 5x for BTC/ETH
+			maxPositionValueRatio = 5.0 // Default: 5x for BTC/ETH and XYZ assets
 		}
 	} else {
 		maxPositionValueRatio = riskControl.AltcoinMaxPositionValueRatio