feat: add hierarchical routing — groups can target other groups

RouteToConcrete() recursively resolves group chains until a concrete
model is reached, with cycle detection and max depth (10) guard.

Example: all-purpose -> fast-flow -> deepseek-v4-flash
The dashboard log shows the full chain: 'deepseek-v4-flash (hierarchical:
fast-flow (default (first target)) -> deepseek-v4-flash (default (first target)))'

internal/router/router.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"strings"
 
 	"gophergate/internal/db"
 )
@@ -76,6 +77,49 @@ func (r *Router) Route(ctx context.Context, groupID string, userMessage string)
 	}
 }
 
+// RouteToConcrete resolves a model name to a concrete model, following group
+// chains recursively until a non-group target is reached. Returns the original
+// name unchanged if it is not a group.
+func (r *Router) RouteToConcrete(ctx context.Context, modelID string, userMessage string) (*Decision, error) {
+	const maxDepth = 10
+	visited := make(map[string]bool)
+	current := modelID
+	var chain []*Decision
+
+	for depth := 0; depth < maxDepth; depth++ {
+		if !r.IsGroup(current) {
+			// Build a composite reason showing the chain traversed
+			reason := "direct"
+			if len(chain) > 0 {
+				parts := make([]string, len(chain))
+				for i, d := range chain {
+					parts[i] = d.SelectedModel + " (" + d.Reason + ")"
+				}
+				reason = strings.Join(parts, " -> ")
+			}
+			return &Decision{
+				SelectedModel: current,
+				Strategy:      "hierarchical",
+				Reason:        reason,
+			}, nil
+		}
+
+		if visited[current] {
+			return nil, fmt.Errorf("routing cycle detected: group %s already visited", current)
+		}
+		visited[current] = true
+
+		decision, err := r.Route(ctx, current, userMessage)
+		if err != nil {
+			return nil, err
+		}
+		chain = append(chain, decision)
+		current = decision.SelectedModel
+	}
+
+	return nil, fmt.Errorf("routing depth exceeded: reached max depth of %d", maxDepth)
+}
+
 // Reload replaces the group definitions without recreating the router.
 func (r *Router) Reload(groups []db.ModelGroup) {
 	r.groups = make(map[string]db.ModelGroup)