更新环境配置示例，添加 GRVT 相关的 API 凭证和选项，增强文档以支持新的交易所适配器，确保用户能够正确配置和使用 GRVT 交易功能。

.env.example

@@ -1,3 +1,6 @@
+# Exchange selection
+EXCHANGE=aster                       # Pick aster (default) or grvt
+
 # Aster API credentials
 ASTER_API_KEY=
 ASTER_API_SECRET=
@@ -30,4 +33,17 @@ MAKER_BID_OFFSET=0                      # Bid quote offset from top bid (USDT)
 MAKER_ASK_OFFSET=0                      # Ask quote offset from top ask (USDT)
 MAKER_REFRESH_INTERVAL_MS=1500          # Maker refresh cadence (ms)
 MAKER_MAX_CLOSE_SLIPPAGE_PCT=0.05       # Maker close slippage guard (fallbacks to MAX_CLOSE_SLIPPAGE_PCT)
-MAKER_PRICE_TICK=0.1                    # Maker price tick size (defaults to PRICE_TICK)
+MAKER_PRICE_TICK=0.1                    # Maker price tick size (defaults to PRICE_TICK)
+
+# GRVT authentication (set when EXCHANGE=grvt)
+GRVT_API_KEY=
+GRVT_API_SECRET=
+GRVT_SUB_ACCOUNT_ID=
+GRVT_INSTRUMENT=BTC_USDT_Perp
+GRVT_SYMBOL=BTCUSDT
+GRVT_ENV=prod
+
+# Optional advanced overrides
+# GRVT_COOKIE="gravity=..."            # Pre-provisioned session cookie (auto-refresh uses API key when absent)
+# GRVT_ACCOUNT_ID=                      # Populated automatically after login
+# GRVT_SIGNER_PATH=./grvt-signer.cjs    # Custom signature provider module