feat: add router package with heuristic strategy

internal/router/heuristic.go

@@ -0,0 +1,73 @@
+package router
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+
+	"gophergate/internal/db"
+)
+
+// HeuristicRule defines a pattern-based routing rule.
+type HeuristicRule struct {
+	Pattern       string `json:"pattern"`
+	TargetIdx     int    `json:"target"`
+	CaseSensitive bool   `json:"case_sensitive,omitempty"`
+}
+
+func routeHeuristic(group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	selected := targets[0]
+	reason := "default (first target)"
+
+	// If heuristic_rules is set, use them
+	if group.HeuristicRules != nil && *group.HeuristicRules != "" {
+		var rules []HeuristicRule
+		if err := json.Unmarshal([]byte(*group.HeuristicRules), &rules); err == nil {
+			searchMsg := userMessage
+			for _, rule := range rules {
+				pattern := rule.Pattern
+				msg := searchMsg
+				if !rule.CaseSensitive {
+					pattern = strings.ToLower(pattern)
+					msg = strings.ToLower(msg)
+				}
+				if strings.Contains(msg, pattern) {
+					if rule.TargetIdx >= 0 && rule.TargetIdx < len(targets) {
+						selected = targets[rule.TargetIdx]
+						reason = "matched heuristic rule: " + rule.Pattern
+						break
+					}
+				}
+			}
+		}
+	}
+
+	// Built-in fallback heuristics
+	if reason == "default (first target)" && len(targets) > 1 {
+		msgLower := strings.ToLower(userMessage)
+		complexIndicators := []string{
+			"step by step", "explain in detail", "reason through",
+			"think carefully", "analyze", "debug", "write code",
+			"implement", "refactor", "architecture",
+		}
+		for _, indicator := range complexIndicators {
+			if strings.Contains(msgLower, indicator) {
+				selected = targets[len(targets)-1]
+				reason = "complex task indicator: " + indicator
+				break
+			}
+		}
+	}
+
+	return &Decision{
+		SelectedModel: selected,
+		Strategy:      "heuristic",
+		Reason:        reason,
+	}, nil
+}
+
+// routeClassifier is a stub — real implementation in classifier.go (Task 3).
+// Falls back to heuristic routing for now.
+func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	return routeHeuristic(group, targets, userMessage)
+}