feat: add oi_low coin source for short opportunities

- Add GetOILowPositions/GetOILowSymbols in oi.go
- Add UseOILow/OILowLimit config fields
- Add oi_low case in GetCandidateCoins
- Support oi_low in mixed mode
- Update source tag formatting

store/strategy.go

@@ -97,7 +97,7 @@ type PromptSectionsConfig struct {
 
 // CoinSourceConfig coin source configuration
 type CoinSourceConfig struct {
-	// source type: "static" | "ai500" | "oi_top" | "mixed"
+	// source type: "static" | "ai500" | "oi_top" | "oi_low" | "mixed"
 	SourceType string `json:"source_type"`
 	// static coin list (used when source_type = "static")
 	StaticCoins []string `json:"static_coins,omitempty"`
@@ -107,10 +107,14 @@ type CoinSourceConfig struct {
 	UseAI500 bool `json:"use_ai500"`
 	// AI500 coin pool maximum count
 	AI500Limit int `json:"ai500_limit,omitempty"`
-	// whether to use OI Top
+	// whether to use OI Top (持仓增加榜，适合做多)
 	UseOITop bool `json:"use_oi_top"`
 	// OI Top maximum count
 	OITopLimit int `json:"oi_top_limit,omitempty"`
+	// whether to use OI Low (持仓减少榜，适合做空)
+	UseOILow bool `json:"use_oi_low"`
+	// OI Low maximum count
+	OILowLimit int `json:"oi_low_limit,omitempty"`
 	// Note: API URLs are now built automatically using NofxOSAPIKey from IndicatorConfig
 }