ax(node): rename nm to manager in NewNodeManagerWithPaths for predictable naming

AX Principle 1 — nm abbreviates NodeManager, requiring mapping overhead.
Rename to manager and update the NewNodeManager usage-example comment to match.

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

pkg/node/identity.go

@@ -79,9 +79,9 @@ type NodeManager struct {
 	mutex sync.RWMutex
 }
 
-// nm, err := node.NewNodeManager()
+// nodeManager, err := node.NewNodeManager()
 // if err != nil { return err }
-// if !nm.HasIdentity() { nm.GenerateIdentity("my-node", RoleDual) }
+// if !nodeManager.HasIdentity() { nodeManager.GenerateIdentity("my-node", RoleDual) }
 func NewNodeManager() (*NodeManager, error) {
 	keyPath, err := xdg.DataFile("lethean-desktop/node/private.key")
 	if err != nil {
@@ -99,18 +99,18 @@ func NewNodeManager() (*NodeManager, error) {
 // NewNodeManagerWithPaths creates a NodeManager with custom paths.
 // This is primarily useful for testing to avoid xdg path caching issues.
 func NewNodeManagerWithPaths(keyPath, configPath string) (*NodeManager, error) {
-	nm := &NodeManager{
+	manager := &NodeManager{
 		keyPath:    keyPath,
 		configPath: configPath,
 	}
 
 	// Try to load existing identity
-	if err := nm.loadIdentity(); err != nil {
+	if err := manager.loadIdentity(); err != nil {
 		// Identity doesn't exist yet, that's ok
-		return nm, nil
+		return manager, nil
 	}
 
-	return nm, nil
+	return manager, nil
 }
 
 // if nm.HasIdentity() { return nm.GetIdentity() }