feat: Add download support for xhs, twi, bilibili, zhihu (#22)

* feat: add download support for images, videos, and articles

Add comprehensive download functionality to OpenCLI with support for
multiple platforms and content types.

- Add `src/download/index.ts`: HTTP download with progress, yt-dlp
  wrapper for video platforms, cookie export to Netscape format for
  authenticated downloads
- Add `src/download/progress.ts`: Terminal progress bars, multi-file
  download tracker with status summary
- Add `src/pipeline/steps/download.ts`: New `download` pipeline step
  for declarative YAML pipelines

- Register `download` step in executor.ts
- Add template filters: `slugify`, `sanitize`, `ext`, `basename` for
  filename templating

- `xiaohongshu download`: Download images and videos from notes
- `bilibili download`: Download videos using yt-dlp with cookie auth
- `twitter download`: Download media from user timeline or single tweet
- `zhihu download`: Export articles to Markdown with optional image
  download

```yaml
pipeline:
  - download:
      url: ${{ item.imageUrl }}
      dir: ./downloads
      filename: ${{ item.title | sanitize }}.jpg
      concurrency: 5
      skip_existing: true
      use_ytdlp: false
      type: auto  # auto|image|video|document
```

- Concurrent downloads with configurable parallelism
- Progress bars with file size display
- Skip existing files option
- Cookie forwarding for authenticated downloads
- yt-dlp integration for video platforms (YouTube, Bilibili, Twitter)
- HTML to Markdown conversion for article export

- yt-dlp: Required for video downloads from streaming platforms
- ffmpeg: Optional for video format conversion

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* docs: add download support documentation

- Add Download Support section to both README.md and README.zh-CN.md
- Document supported platforms: Xiaohongshu, Bilibili, Twitter, Zhihu
- Include prerequisites (yt-dlp installation)
- Add usage examples for all download commands
- Document the `download` pipeline step for YAML adapters
- Update built-in commands table with new `download` commands

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix: preserve zhihu ordered list content

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: jackwener <jakevingoo@gmail.com>