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/channels/email.spec.ts

@@ -0,0 +1,38 @@
+import { test, expect, channels, expectUnchanged, Mailbox, AI_REPLY } from '@e2e'
+
+test('an email reply stays in the customer\'s thread @channels', async ({ agent, llm, adminApi, request }) => {
+  await llm.script(agent, (s) => s.otherwise().reply('Happy to help with that.'))
+
+  const chat = (await channels.Email({ api: adminApi, request, agentId: agent.id })).chatWith('Tess')
+  await chat.send('Can I change my delivery address?')
+
+  const mailbox = new Mailbox(request)
+  await expect.poll(async () => (await mailbox.messagesTo(chat.address)).length, AI_REPLY).toBe(1)
+  // One question, one model call, one email — a duplicate would arrive later.
+  expect(await llm.requests(agent)).toHaveLength(1)
+  await expectUnchanged('emails to the customer', async () => (await mailbox.messagesTo(chat.address)).length)
+  const [reply] = await mailbox.messagesTo(chat.address)
+  expect(reply.subject).toBe(`Re: ${chat.subject}`)
+  // In-Reply-To is what makes mail clients file the answer under the question.
+  expect((await mailbox.headers(reply.id))['In-Reply-To']).toEqual([chat.lastMessageId])
+})
+
+test('a customer\'s follow-up email continues the same conversation @channels', async ({ agent, llm, adminApi, request }) => {
+  await llm.script(agent, (s) => s
+    .when(/tuesday/i).reply('Tuesday works — I have noted it.')
+    .otherwise().reply('Happy to help with that. Which day suits you?'))
+
+  const chat = (await channels.Email({ api: adminApi, request, agentId: agent.id })).chatWith('Tess')
+  await chat.send('Can I change my delivery day?')
+  await expect.poll(() => chat.replies(), AI_REPLY).toHaveLength(1)
+  await chat.send('Tuesday please')
+  await expect.poll(() => chat.replies(), AI_REPLY).toEqual([
+    'Happy to help with that. Which day suits you?',
+    'Tuesday works — I have noted it.',
+  ])
+
+  // One conversation for this sender, holding both exchanges — not two.
+  const conversations = await adminApi.get<Array<{ message_count: number }>>('/chats/recent', { params: { customer_email: chat.address } })
+  expect(conversations).toHaveLength(1)
+  expect(conversations[0].message_count).toBe(4)
+})