refactor: bind NodeService to a user and expose User() on the interface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

service/node_service.go

@@ -3,12 +3,19 @@ package service
 import (
 	"axolotl/models"
 	"axolotl/store"
+	"fmt"
 )
 
 // NodeService is the single entry point for all node operations.
 // All data-model integrity rules are enforced here; callers cannot produce
 // invalid state by interacting with this interface alone.
+//
+// Every NodeService instance is bound to a specific user (see User()).
+// GetNodeService returns an error when no user is configured.
 type NodeService interface {
+	// User returns the name/ID of the user this service instance acts on behalf of.
+	User() string
+
 	// Query
 	GetByID(id string) (*models.Node, error)
 	List(filter ListFilter) ([]*models.Node, error)
@@ -69,9 +76,13 @@ func InitNodeService(path string) error {
 }
 
 func GetNodeService(cfg Config) (NodeService, error) {
+	user := cfg.GetUser()
+	if user == "" {
+		return nil, fmt.Errorf("no user configured: run 'ax user set <username>' first")
+	}
 	st, err := store.FindAndOpenSQLiteStore()
 	if err != nil {
 		return nil, err
 	}
-	return &nodeServiceImpl{store: st, userID: cfg.GetUser()}, nil
+	return &nodeServiceImpl{store: st, userID: user}, nil
 }

service/node_service_impl.go

@@ -26,6 +26,8 @@ func mentions(t string) []string {
 	return slices.Collect(maps.Keys(seen))
 }
 
+func (s *nodeServiceImpl) User() string { return s.userID }
+
 // --- Validation ---
 
 var (