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/handover/attachments.spec.ts

@@ -0,0 +1,30 @@
+import { test, expect, env, unique, AI_REPLY } from '@e2e'
+
+test.describe('attachments', () => {
+  test.use({ agentSettings: { transferToHuman: true, allowAttachments: true } })
+
+  test('a customer sends a file to the teammate, who can open it — and only through a signed link @handover', async ({ agent, widget, inbox, llm, request }) => {
+    await llm.script(agent, (s) => s.otherwise().handOver('Let me connect you with a teammate.'))
+    await widget.embed()
+    await widget.open()
+    await widget.send('I need a person')
+    await widget.shareContactDetails()
+    await inbox.goto()
+    await inbox.openConversationWith(widget.customer.name)
+    await inbox.takeOver()
+    await inbox.reply('Please send me the receipt.')
+    await expect(widget.lastAgentMessage()).toHaveText('Please send me the receipt.', AI_REPLY)
+
+    const content = `Receipt ${unique('R')} — total 42.00`
+    await widget.sendFile({ name: 'receipt.txt', mimeType: 'text/plain', content }, 'Here it is')
+
+    const link = inbox.attachment('receipt.txt')
+    await expect(link).toBeVisible(AI_REPLY)
+    const href = await link.getAttribute('href')
+    expect((await (await inbox.page.request.get(href!)).text())).toBe(content)
+
+    // The stored file is not reachable without the signature.
+    const unsigned = new URL(href!, 'http://x').pathname
+    expect((await request.get(`${env.backendUrl}${unsigned}`)).status()).toBe(403)
+  })
+})