feat: classifier bucket mapping + dispatcher seed group

Classifier: When complexity_threshold is set (e.g. 10), uses it as the
rating scale and maps ratings proportionally to target buckets instead
of 1:1. Formula: idx = rating * len(targets) / (threshold + 1).

With threshold=10 and 3 targets: 1-3→target[0], 4-7→target[1], 8-10→target[2].

Seed: Added 'dispatcher' group (classifier, threshold=10, selector=deepseek-v4-flash)
that auto-routes to fast-flow/standard-pro/heavy-logic by complexity score.

Combined with hierarchical routing, this enables two-level dispatch:
  dispatcher scores 1-10 → routes to tier group → tier picks concrete model.

internal/router/classifier.go

@@ -16,11 +16,19 @@ const classifierSystemPrompt = `You are a task complexity classifier. Rate the f
 Reply with ONLY the number. No explanation.`
 
 func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, userMessage string) (*Decision, error) {
+	// Determine the rating scale
 	maxRating := len(targets)
 	if maxRating < 2 {
 		maxRating = 2
 	}
 
+	// When complexity_threshold is set, use it as a wider scale (e.g., 1-10)
+	// and map ratings proportionally to target buckets.
+	bucketMode := group.ComplexityThreshold != nil && *group.ComplexityThreshold > 0
+	if bucketMode {
+		maxRating = *group.ComplexityThreshold
+	}
+
 	prompt := fmt.Sprintf(classifierSystemPrompt, maxRating, maxRating)
 	ratingStr, err := classify(ctx, getSelectorModel(group, targets), prompt, userMessage)
 	if err != nil {
@@ -36,7 +44,18 @@ func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.Mode
 		rating = maxRating
 	}
 
-	idx := rating - 1 // 0-based index into targets
+	var idx int
+	if bucketMode {
+		// Proportional mapping: wider scale → N target buckets
+		// e.g., threshold=10, 3 targets: 1-3→0, 4-7→1, 8-10→2
+		idx = rating * len(targets) / (maxRating + 1)
+		if idx >= len(targets) {
+			idx = len(targets) - 1
+		}
+	} else {
+		idx = rating - 1 // 1:1 mapping
+	}
+
 	return &Decision{
 		SelectedModel: targets[idx],
 		Strategy:      "classifier",