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/test_all_starred.py

@@ -1,7 +1,7 @@
 """Tests for --all-starred flag behavior (issue #225)."""
 
 import pytest
-from unittest.mock import Mock, patch
+from unittest.mock import patch
 
 from github_backup import github_backup
 
@@ -12,58 +12,14 @@ class TestAllStarredCloning:
     Issue #225: --all-starred should clone starred repos without requiring --repositories.
     """
 
-    def _create_mock_args(self, **overrides):
-        """Create a mock args object with sensible defaults."""
-        args = Mock()
-        args.user = "testuser"
-        args.output_directory = "/tmp/backup"
-        args.include_repository = False
-        args.include_everything = False
-        args.include_gists = False
-        args.include_starred_gists = False
-        args.all_starred = False
-        args.skip_existing = False
-        args.bare_clone = False
-        args.lfs_clone = False
-        args.no_prune = False
-        args.include_wiki = False
-        args.include_issues = False
-        args.include_issue_comments = False
-        args.include_issue_events = False
-        args.include_pulls = False
-        args.include_pull_comments = False
-        args.include_pull_commits = False
-        args.include_pull_details = False
-        args.include_labels = False
-        args.include_hooks = False
-        args.include_milestones = False
-        args.include_security_advisories = False
-        args.include_releases = False
-        args.include_assets = False
-        args.include_attachments = False
-        args.incremental = False
-        args.incremental_by_files = False
-        args.github_host = None
-        args.prefer_ssh = False
-        args.token_classic = None
-        args.token_fine = None
-        args.as_app = False
-        args.osx_keychain_item_name = None
-        args.osx_keychain_item_account = None
-
-        for key, value in overrides.items():
-            setattr(args, key, value)
-
-        return args
-
     @patch('github_backup.github_backup.fetch_repository')
     @patch('github_backup.github_backup.get_github_repo_url')
-    def test_all_starred_clones_without_repositories_flag(self, mock_get_url, mock_fetch):
+    def test_all_starred_clones_without_repositories_flag(self, mock_get_url, mock_fetch, create_args):
         """--all-starred should clone starred repos without --repositories flag.
 
         This is the core fix for issue #225.
         """
-        args = self._create_mock_args(all_starred=True)
+        args = create_args(all_starred=True)
         mock_get_url.return_value = "https://github.com/otheruser/awesome-project.git"
 
         # A starred repository (is_starred flag set by retrieve_repositories)
@@ -88,9 +44,9 @@ class TestAllStarredCloning:
 
     @patch('github_backup.github_backup.fetch_repository')
     @patch('github_backup.github_backup.get_github_repo_url')
-    def test_starred_repo_not_cloned_without_all_starred_flag(self, mock_get_url, mock_fetch):
+    def test_starred_repo_not_cloned_without_all_starred_flag(self, mock_get_url, mock_fetch, create_args):
         """Starred repos should NOT be cloned if --all-starred is not set."""
-        args = self._create_mock_args(all_starred=False)
+        args = create_args(all_starred=False)
         mock_get_url.return_value = "https://github.com/otheruser/awesome-project.git"
 
         starred_repo = {
@@ -111,9 +67,9 @@ class TestAllStarredCloning:
 
     @patch('github_backup.github_backup.fetch_repository')
     @patch('github_backup.github_backup.get_github_repo_url')
-    def test_non_starred_repo_not_cloned_with_only_all_starred(self, mock_get_url, mock_fetch):
+    def test_non_starred_repo_not_cloned_with_only_all_starred(self, mock_get_url, mock_fetch, create_args):
         """Non-starred repos should NOT be cloned when only --all-starred is set."""
-        args = self._create_mock_args(all_starred=True)
+        args = create_args(all_starred=True)
         mock_get_url.return_value = "https://github.com/testuser/my-project.git"
 
         # A regular (non-starred) repository
@@ -135,9 +91,9 @@ class TestAllStarredCloning:
 
     @patch('github_backup.github_backup.fetch_repository')
     @patch('github_backup.github_backup.get_github_repo_url')
-    def test_repositories_flag_still_works(self, mock_get_url, mock_fetch):
+    def test_repositories_flag_still_works(self, mock_get_url, mock_fetch, create_args):
         """--repositories flag should still clone repos as before."""
-        args = self._create_mock_args(include_repository=True)
+        args = create_args(include_repository=True)
         mock_get_url.return_value = "https://github.com/testuser/my-project.git"
 
         regular_repo = {