feat(markdown): add CommonMark-aware extraction

[luoxiaojian]

Summary

Improve Markdown extraction by replacing ad-hoc heading and fence detection with CommonMark and GitHub Flavored Markdown block parsing.

• Parse ATX and Setext headings according to CommonMark rules.
• Respect fenced code, HTML blocks, lists, blockquotes, footnotes, and GFM tables during section discovery.
• Prefer parsed block boundaries when splitting large Markdown documents.
• Treat leading YAML front matter as a document envelope and exclude it from content chunks.
• Normalize bounded top-level scalar and scalar-list front matter fields into generic frontMatter metadata.
• Prioritize high-signal metadata such as titles, descriptions, names, and tags in embedding input.
• Persist generic front matter metadata and bump the workspace index version.

Motivation

A YAML sequence item immediately before a closing front matter delimiter can resemble a Markdown Setext heading:

---
api_name:
- ExampleApi
---