feat(xianyu): add inbox, messages, and reply commands

[jun0315]

Description

Adds Xianyu private-message support:

• Add xianyu inbox to list recent IM conversations with unread and unread_count
• Add xianyu messages to read recent messages from a specific conversation
• Add xianyu reply to reply to a specific conversation
• Support --rank for messages / reply so commands can operate on visible inbox rows when Xianyu does not expose itemId / peerUserId
• Refactor shared Xianyu IM DOM helpers into clis/xianyu/im.js
• Update docs, README command list, manifest, and tests

Related issue:

Type of Change

• Bug fix
• ✨ New feature
• New site adapter
• Documentation
• ♻️ Refactor
• CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Checks run:

npx vitest run --project adapter clis/xianyu
5 files passed, 33 tests passed

npx tsx src/main.ts validate xianyu
opencli validate: PASS
Checked 7 command(s)
npm run check:silent-column-drop -- xianyu/inbox xianyu/messages xianyu/reply xianyu/chat
OK - no new silent-column-drop violations.

xianyu inbox --limit 10 --unread-only true --site-session persistent
Returned only unread conversations.
Returned real conversation messages.

xianyu reply --rank 1 --text "测试消息，请忽略" --site-session persistent
Returned status: sent, and the message was visible when read back.