Deprecate the full-result MCP search tool

[justincasher]

Summary

• retain the MCP search tool so existing clients keep working
• advertise it as deprecated in its title, description, and tool metadata, with search_summary as its replacement
• instruct agents to always use search_summary followed by per-field retrieval tools
• update the package, plugin, MCP, and data-model documentation
• add coverage for the tool metadata exposed through MCP discovery

The Python ApiClient.search() method and CLI lean-explore search command are unchanged. This deprecates only the token-heavy MCP tool.

Verification

• ruff check src/lean_explore/mcp/app.py src/lean_explore/mcp/tools.py tests/mcp/tools_test.py
• pytest tests/mcp -q (67 passed)

Rollout

Merge this PR before the companion lean-explore-app documentation PR. The app deployment resets the sibling lean-explore checkout to origin/main, so that deployment will pick up this server-tool change.