fix(mem0-plugin): use env var in .mcp.json, fix onboarding MCP check

- .mcp.json: use ${MEM0_API_KEY} instead of ${user_config.MEM0_API_KEY}
  so MCP connects for --plugin-dir users with the key in their shell
- mem0-onboard: check MCP tools availability first, stop early with
  clear instructions if MCP server not connected

mem0-plugin/.mcp.json

@@ -4,7 +4,7 @@
       "type": "http",
       "url": "https://mcp.mem0.ai/mcp/",
       "headers": {
-        "Authorization": "Token ${user_config.MEM0_API_KEY}"
+        "Authorization": "Token ${MEM0_API_KEY}"
       }
     }
   }