refactor(ui): extract stdio plugin package

Move the readline front door from stdio-agent into @deepseek-ai/dsh-stdio, keeping the loader shape and the stdio coverage with the new package.

knip.json

@@ -85,6 +85,10 @@
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]
     },
+    "packages/ui/stdio": {
+      "entry": ["tests/**/*.spec.ts"],
+      "project": ["src/**/*.ts", "tests/**/*.ts"]
+    },
     "packages/ui/jsonrpc-agent": {
       "project": ["src/**/*.ts"]
     },