feat: implement advanced condition-based heuristic model routing

Upgrades the routing engine to support tag, token limit, multimodal, reasoning, and tool calling conditions. Adds unit tests for the new routing features.

internal/router/classifier.go

@@ -15,7 +15,7 @@ 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) {
+func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.ModelGroup, targets []string, routeCtx *RouteContext) (*Decision, error) {
 	// Determine the rating scale
 	maxRating := len(targets)
 	if maxRating < 2 {
@@ -30,10 +30,14 @@ func routeClassifier(ctx context.Context, classify ClassifierFunc, group db.Mode
 	}
 
 	prompt := fmt.Sprintf(classifierSystemPrompt, maxRating, maxRating)
-	ratingStr, err := classify(ctx, getSelectorModel(group, targets), prompt, userMessage)
+	userMsg := ""
+	if routeCtx != nil {
+		userMsg = routeCtx.UserMessage
+	}
+	ratingStr, err := classify(ctx, getSelectorModel(group, targets), prompt, userMsg)
 	if err != nil {
 		// Classifier failed — fall back to heuristic
-		return routeHeuristic(group, targets, userMessage)
+		return routeHeuristic(group, targets, routeCtx)
 	}
 
 	rating, err := strconv.Atoi(strings.TrimSpace(ratingStr))