feat: add CodeSignalsSearchTool for code-level repository search

[rohit-sahoo]

Summary

Add CodeSignalsSearchTool as a fallback for searching repositories when README-based search (RepositorySearchTool) returns insufficient results. Searches actual source code content to find repos with minimal documentation.

What have I done

• Added CodeSignalsSearchTool that searches LLM-extracted code signals (function names, class names, imports, code summaries)
• Implemented _extract_summary() to extract only "Code Summary:" sections, significantly reducing context size
• Added safety limits: max 6 results, default is 5
• Integrated with existing MCP server via @mcp_tool decorator

How did I do

• Tool calls /api/code_signals/search endpoint with hybrid search (vector + keyword)
• Parses response and extracts summaries from code_signals field
• Returns CodeSignalsHit objects with repo_id, repo_url, content, and score

How to test

# Run tests
uv run pytest tests/tools/code_search/test_code_signals_search.py