feat(anthropic): add anthropic article download command

[CenturySee]

Description

Adds opencli anthropic download for exporting Anthropic articles to local Markdown.

The command supports article URLs under https://www.anthropic.com/, including engineering, news, and research/index-style pages. It extracts the article title, date, description, and body from the rendered page, writes YAML frontmatter, saves Markdown as YYYY-MM-DD-title.md, downloads cover/body images into a sibling _assets directory, and rewrites Markdown image links to local relative paths.

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

node --check clis/anthropic/download.js
npm run build-manifest
npm run docs:build
npm run dev -- anthropic download --url "https://www.anthropic.com/news/claude-haiku-4-5" --output C:\tmp\opencli-anthropic-pr-ready --download-images false

- title: Introducing Claude Haiku 4.5
  date: '2025-10-15'
  status: success
  images: 0
  saved: C:\tmp\opencli-anthropic-pr-ready\2025-10-15-Introducing_Claude_Haiku_4.5.md
  assets: ''