refactor(mcp): remove standalone HTTP mode, keep only built-in endpoint and stdio

- MCP Server is now only available via built-in /mcp endpoint or stdio
- Remove --http flag and standalone HTTP server code
- Simplify documentation and frontend settings

web/src/pages/Settings.tsx

@@ -322,11 +322,7 @@ export default function Settings() {
   "mcpServers": {
     "taskpool": {
       "command": "taskpool",
-      "args": ["mcp"],
-      "env": {
-        "TASKPOOL_URL": "${window.location.origin}",
-        "TASKPOOL_TOKEN": "${siteForm.openapi_token || '在设置页生成 Token'}"
-      }
+      "args": ["mcp"]
     }
   }
 }`}</pre>