refactor: move search_usage to utils/searchusage, remove brave stub

- Rename agent/tools/search_usage.py → utils/searchusage.py
  (not an LLM tool, matches utils/ naming convention)
- Remove redundant _fetch_brave_usage — handled by else branch
- Move test to tests/utils/test_searchusage.py

Made-with: Cursor

tests/utils/test_searchusage.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import pytest
 from unittest.mock import AsyncMock, MagicMock, patch
 
-from nanobot.agent.tools.search_usage import (
+from nanobot.utils.searchusage import (
     SearchUsageInfo,
     _parse_tavily_usage,
     fetch_search_usage,