ax(mining): replace prose comment with usage example on NodeService struct

AX Principle 2 — comments must show HOW with real values, not restate what
the type name already says. Replace the description-style comment with a
concrete two-line call sequence showing construction and route registration.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/node_service.go

@@ -10,7 +10,8 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
-// NodeService handles P2P node-related API endpoints.
+// ns, err := NewNodeService()
+// ns.SetupRoutes(router.Group("/api/v1/mining"))
 type NodeService struct {
 	nodeManager  *node.NodeManager
 	peerRegistry *node.PeerRegistry