ax(mining): replace implementation-describing comment with call-site example on saveUnlocked

AX Principle 2: comments show HOW to call, not WHAT the body does.
The old comment restated the lock/defer mechanics; the new comment
shows the caller context (Save and Update) so an agent knows when
saveUnlocked is the right call.

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

pkg/mining/repository.go

@@ -83,7 +83,7 @@ func (r *FileRepository[T]) Save(data T) error {
 	return r.saveUnlocked(data)
 }
 
-// r.mutex.Lock(); defer r.mutex.Unlock(); return r.saveUnlocked(data)
+// return r.saveUnlocked(data)  // called by Save and Update while mutex is held
 func (r *FileRepository[T]) saveUnlocked(data T) error {
 	dir := filepath.Dir(r.path)
 	if err := os.MkdirAll(dir, 0755); err != nil {