Add titles to public MCP tools

[justincasher]

Summary

• add human-readable titles to search_summary and all six per-field retrieval tools
• retain the deprecated compatibility title on search
• require every registered MCP tool to advertise a non-empty title in tests

Anthropic currently requires every Connectors Directory tool to include a title plus the applicable read-only or destructive annotations. The live endpoint already has the annotations, but only the deprecated search tool had a title.

Verification

• .venv/bin/pytest tests/mcp/tools_test.py -q (48 passed)
• .venv/bin/ruff check src/lean_explore/mcp/tools.py tests/mcp/tools_test.py
• .venv/bin/ruff format --check src/lean_explore/mcp/tools.py tests/mcp/tools_test.py
• git diff --check