feat(11-01): create stealth fingerprint masking module

- Add StealthConfig Pydantic model with all playwright-stealth options
- Add apply_stealth_sync/apply_stealth_async functions for browser context
- Add get_default_stealth_config convenience function
- Add 9 unit tests covering config defaults, stealth application, and disabled mode
- Update conftest.py with stealth_config and mock_browser_context fixtures

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

tests/test_crawler/conftest.py

@@ -102,3 +102,18 @@ def sample_article_url():
 def sample_invalid_url():
     """Sample invalid URL for testing error handling."""
     return "https://www.example.com/some-page"
+
+
+@pytest.fixture
+def stealth_config():
+    """Default StealthConfig for testing."""
+    from kb_workflow.crawler.stealth import StealthConfig
+    return StealthConfig()
+
+
+@pytest.fixture
+def mock_browser_context():
+    """Mock Playwright BrowserContext for stealth testing."""
+    context = MagicMock()
+    context.add_init_script = MagicMock()
+    return context