fix: clean up omnivoice import guard and __init__ error masking

Remove OmniVoice = None fallback in nodes/loader.py so missing omnivoice
gives a clear ImportError instead of a confusing AttributeError. Restore
__init__.py to clean form without the try/except that silently swallowed
real import errors. Add omnivoice mock to conftest.py and register a
pytest plugin that prevents pytest from treating the project root as a
Package node (which would try to import __init__.py outside a package
context and fail on the relative import).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,5 @@
+import sys
+from unittest.mock import MagicMock
+
+# Mock omnivoice so it can be imported in tests without being installed
+sys.modules.setdefault("omnivoice", MagicMock())