ax(mining): rename fn to modifier in UpdateMinersConfig

`fn` is a single-letter abbreviation — AX principle 1 requires predictable
names over short names. `modifier` describes the parameter's role clearly.
Comment updated to a usage example per principle 2.

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

pkg/mining/config_manager.go

@@ -107,8 +107,8 @@ func SaveMinersConfig(configuration *MinersConfig) error {
 }
 
 // UpdateMinersConfig atomically loads, modifies, and saves the miners config.
-// This prevents race conditions in read-modify-write operations.
-func UpdateMinersConfig(fn func(*MinersConfig) error) error {
+// UpdateMinersConfig(func(c *MinersConfig) error { c.Miners = append(c.Miners, entry); return nil })
+func UpdateMinersConfig(modifier func(*MinersConfig) error) error {
 	configMu.Lock()
 	defer configMu.Unlock()
 
@@ -139,7 +139,7 @@ func UpdateMinersConfig(fn func(*MinersConfig) error) error {
 	}
 
 	// Apply the modification
-	if err := fn(&configuration); err != nil {
+	if err := modifier(&configuration); err != nil {
 		return err
 	}