Refactor test fixtures to use shared create_args helper

Uses the real parse_args() function to get CLI defaults, so when
new arguments are added they're automatically available to all tests.

Changes:
- Add tests/conftest.py with create_args fixture
- Update 8 test files to use shared fixture
- Remove duplicate _create_mock_args methods
- Remove redundant @pytest.fixture mock_args definitions

This eliminates the need to update multiple test files when
adding new CLI arguments.

tests/conftest.py

@@ -0,0 +1,25 @@
+"""Shared pytest fixtures for github-backup tests."""
+
+import pytest
+
+from github_backup.github_backup import parse_args
+
+
+@pytest.fixture
+def create_args():
+    """Factory fixture that creates args with real CLI defaults.
+
+    Uses the actual argument parser so new CLI args are automatically
+    available with their defaults - no test updates needed.
+
+    Usage:
+        def test_something(self, create_args):
+            args = create_args(include_releases=True, user="myuser")
+    """
+    def _create(**overrides):
+        # Use real parser to get actual defaults
+        args = parse_args(["testuser"])
+        for key, value in overrides.items():
+            setattr(args, key, value)
+        return args
+    return _create