Rename core -> sdk (plugin SDK, self-documenting)

tests/test_plugin_enhancements.py

@@ -2,8 +2,8 @@
 import pytest, asyncio, os, sys
 sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
 
-from core.plugin_status import PluginStatus
-from core.plugin_error import (
+from sdk.plugin_status import PluginStatus
+from sdk.plugin_error import (
     PluginError, PluginLoadError, PluginConfigError,
     PluginPermissionError, PluginCommandError
 )