fix(mem0-plugin): revert .mcp.json to ${MEM0_API_KEY} env var

${user_config.*} interpolation not supported in HTTP-type MCP server
headers — only works for subprocess-based servers. Revert to shell
env var reference. The userConfig + resolve_api_key() fallback chain
still fixes hook scripts for issue #4876.

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}"
       }
     }
   }