docs(readme): add codegraph_explore to the MCP Tools table (#226)

Add the missing `codegraph_explore` row to the 'MCP server exposes these tools' table — tools.ts exports 9 tools but the table listed 8. (The PR's Node-badge bump was dropped: that badge was replaced by 'Node.js bundled · none required' when the runtime became self-contained.)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>