feat(xiaoyuzhou): add episode download and transcript support

[eeee0717]

Description

• add opencli xiaoyuzhou download for episode audio downloads from public episode payloads
• add opencli xiaoyuzhou transcript for transcript JSON/text downloads with local credential-file auth
• add Xiaoyuzhou auth helpers and adapter tests
• update manifest and user-facing docs for the new commands
• remove env credential fallback so the transcript auth contract matches the docs

Related issue:

N/A

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

• npx vitest run clis/xiaoyuzhou/auth.test.js clis/xiaoyuzhou/transcript.test.js clis/xiaoyuzhou/download.test.js
• 3 test files passed, 13 tests passed