feat: add classifier routing strategy with LLM complexity rating

internal/router/classifier.go

@@ -0,0 +1,53 @@
+package router
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"gophergate/internal/db"
+)
+
+const classifierSystemPrompt = `You are a task complexity classifier. Rate the following user message on a scale of 1 to %d, where:
+1 = trivial/simple (basic facts, greetings, simple math)
+%d = highly complex (multi-step reasoning, code generation, architecture design)
+
+Reply with ONLY the number. No explanation.`
+
+func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	maxRating := len(targets)
+	if maxRating < 2 {
+		maxRating = 2
+	}
+
+	prompt := fmt.Sprintf(classifierSystemPrompt, maxRating, maxRating)
+	ratingStr, err := classify(ctx, getSelectorModel(group, targets), prompt, userMessage)
+	if err != nil {
+		// Classifier failed — fall back to heuristic
+		return routeHeuristic(group, targets, userMessage)
+	}
+
+	rating, err := strconv.Atoi(strings.TrimSpace(ratingStr))
+	if err != nil || rating < 1 {
+		rating = 1
+	}
+	if rating > maxRating {
+		rating = maxRating
+	}
+
+	idx := rating - 1 // 0-based index into targets
+	return &Decision{
+		SelectedModel: targets[idx],
+		Strategy:      "classifier",
+		Reason:        fmt.Sprintf("complexity rating: %d/%d", rating, maxRating),
+	}, nil
+}
+
+func getSelectorModel(group db.ModelGroup, targets []string) string {
+	if group.SelectorModel != nil && *group.SelectorModel != "" {
+		return *group.SelectorModel
+	}
+	// Default: use the first (cheapest) target model as the selector
+	return targets[0]
+}

internal/router/heuristic.go

@@ -1,7 +1,6 @@
 package router
 
 import (
-	"context"
 	"encoding/json"
 	"strings"
 
@@ -65,9 +64,3 @@ func routeHeuristic(group db.ModelGroup, targets []string, userMessage string) (
 		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)
-}