docs: 交付文档、维护指引与端到端回归

补齐 README、开发计划、维护交接与 E2E 用例，执行最终验收。

e2e/specs/widget/iframe-snippet.spec.ts

@@ -0,0 +1,14 @@
+import { test, expect, AI_REPLY } from '@e2e'
+
+// The dashboard also offers an iframe snippet: the chat page itself, served by
+// the API. Its socket comes from the API's own origin, which must be allowed
+// whether or not the operator listed it in CORS_ORIGINS.
+
+test('a customer chats through the iframe snippet @widget', async ({ agent, widget, llm }) => {
+  await llm.script(agent, (s) => s.otherwise().reply('Hello from the iframe'))
+
+  await widget.embed({ snippet: 'iframe' })
+  await widget.expectReady()
+  await widget.send('Hi')
+  await expect(widget.lastAgentMessage()).toHaveText('Hello from the iframe', AI_REPLY)
+})