SPARQL query capabilities in agent via MCP

## What

Enable agent to query knowledge graphs using SPARQL via MCP server.

## Why

Agent needs to read existing concepts before adding new ones to avoid duplicates and create proper links.

## How

- Agent calls MCP server's SPARQL tools
- Query existing concepts by label
- Find related concepts
- Build context for new triples

## Component

- [x] agent
- [x] mcp-server