refactor: remove dead and test-only code and the write-only prompt registry (#620)

Removes helpers nothing calls, test-only wrappers and their tests, the unused
config writer, three unused images, and the prompt registry the nodes wrote to
but nothing read, which grew with every run.

tests/test_split_node.py

@@ -28,7 +28,6 @@ class TestSplitNode(unittest.TestCase):
     def test_split_encodes_composition_and_persists_main_text_with_metadata(self):
         node = PromptManagerSplit.__new__(PromptManagerSplit)
         node.logger = logging.getLogger("test.split")
-        node.comfyui_integration = unittest.mock.Mock()
         node._inject_lora_trigger_words = lambda text: text
         node._track_prompt_execution = unittest.mock.Mock(return_value=42)
         clip = unittest.mock.Mock()