Improve social post markdown export fidelity

[shellen]

Problem

Social posts saved as .md files lack faithful representation of the original post:

• Bluesky posts: title is "Untitled", body is raw text without author attribution
• X posts from bookmark feeds: title includes full tweet text with "[Private]" prefix and "/ X" suffix, body is weird favicon image links and screenshots
• Mastodon posts: similar issues to Bluesky

Desired behavior

Social post .md files should include:

• Clean title: author name + short excerpt (like the viewer does)
• Frontmatter: platform, handle, display name
• Body: properly formatted post text with author byline, similar to formatTweetMarkdown

What needs to happen

• Add Bluesky/Mastodon equivalents of formatTweetMarkdown in extractor.ts
• Use public APIs (bsky.app public API, Mastodon instance API) to fetch structured post data
• Generate clean markdown with author, timestamp, post text, media, quoted posts
• Fix title generation for all social platforms

Context

Twitter has formatTweetMarkdown via fxtwitter API. Bluesky/Mastodon go through generic Defuddle extraction which doesn't understand social post structure.