feat: add web search tool using SearXNG

Add WebSearchTool that queries the self-hosted SearXNG metasearch engine
for current information, documentation, and facts beyond training data.

- Add SEARXNG_URL and SEARXNG_TIMEOUT config settings
- Create WebSearchTool with query, num_results, categories params
- Register web_search tool with explore agent
- Add 10 tests for search functionality

Usage: Agents can now use web_search(query="...") to find current info.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/domains/tools/__init__.py

@@ -5,7 +5,7 @@ All tools inherit from BaseTool and return ToolResult.
 """
 from src.domains.tools.base import BaseTool, ToolResult
 from src.domains.tools.file import ReadFileTool, GlobFilesTool, EditFileTool, WriteFileTool
-from src.domains.tools.search import GrepContentTool
+from src.domains.tools.search import GrepContentTool, WebSearchTool
 from src.domains.tools.shell import BashReadOnlyTool, BashTool
 
 __all__ = [
@@ -16,6 +16,7 @@ __all__ = [
     "EditFileTool",
     "WriteFileTool",
     "GrepContentTool",
+    "WebSearchTool",
     "BashReadOnlyTool",
     "BashTool",
 ]