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>

kb_workflow/crawler/stealth/__init__.py

@@ -0,0 +1,15 @@
+"""Stealth module for browser fingerprint masking."""
+
+from .stealth_config import (
+    StealthConfig,
+    apply_stealth_sync,
+    apply_stealth_async,
+    get_default_stealth_config,
+)
+
+__all__ = [
+    "StealthConfig",
+    "apply_stealth_sync",
+    "apply_stealth_async",
+    "get_default_stealth_config",
+]