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/src/channels/email.ts

@@ -0,0 +1,60 @@
+import { connectEmail, routeToAgent } from '../api/channels'
+import { env } from '../env'
+import { Mailbox } from '../fakes/mailbox'
+import { unique } from '../support/ids'
+import { step } from '../support/step'
+import { Chat, postWebhook, type ChannelChat, type ChannelConnector } from './chat'
+
+/** An email chat, plus what only email has: the subject line every message carries. */
+export interface EmailChat extends ChannelChat {
+  readonly address: string
+  readonly subject: string
+  /** Message-ID of the customer's latest email, which a threaded reply points back at. */
+  readonly lastMessageId: string | undefined
+}
+
+class EmailConversation extends Chat implements EmailChat {
+  readonly address: string
+  readonly subject: string
+  private readonly sent: { messageId?: string }
+
+  constructor(customerName: string, address: string, subject: string, sent: { messageId?: string }, deliver: (text: string) => Promise<void>, read: () => Promise<string[]>) {
+    super('email', customerName, deliver, read)
+    this.address = address
+    this.subject = subject
+    this.sent = sent
+  }
+
+  get lastMessageId(): string | undefined {
+    return this.sent.messageId
+  }
+}
+
+/**
+ * A support inbox fed by an inbound-parse webhook (the per-account URL carries a
+ * secret token, there is no signature). Replies go out over SMTP into the stack's
+ * Mailpit, where the customer's address collects them.
+ */
+export const email: ChannelConnector<EmailChat> = ({ api, request, agentId }) =>
+  step('Connect a support inbox', async () => {
+    const inbox = `${unique('support')}@e2e.example.com`
+    const account = await connectEmail(api, inbox)
+    await routeToAgent(api, account.id, agentId)
+    const token = new URL(account.webhook_url ?? '', env.backendUrl).searchParams.get('token')
+    if (!token) throw new Error(`no token in the email webhook URL ${account.webhook_url}`)
+    const mailbox = new Mailbox(request)
+
+    return {
+      chatWith(customerName) {
+        const address = `${unique('customer').toLowerCase()}@customer.example.com`
+        const subject = `Question from ${customerName}`
+        const sent: { messageId?: string } = {}
+        return new EmailConversation(customerName, address, subject, sent, async (text) => {
+          sent.messageId = `<${unique('msg')}@customer.example.com>`
+          await postWebhook(request, `/webhooks/email/${account.id}?token=${token}`, {
+            json: JSON.stringify({ from: `${customerName} <${address}>`, to: inbox, subject, text, message_id: sent.messageId }),
+          })
+        }, async () => (await mailbox.messagesTo(address)).filter((m) => m.from === inbox).map((m) => m.text.trim()))
+      },
+    }
+  })