docs(typefully): document X Article code blocks

[linuz90]

TL;DR

The Typefully agent skill now teaches agents how to create fenced code blocks in X Articles without corrupting internal backticks, and makes the code-comment limitation explicit.

Ref TYP-7441

Summary

• Document fenced code blocks as supported X Article Markdown, including optional single-token languages and collision-safe outer fences.
• Extend the comprehensive example with code while keeping the nested Markdown fence valid.
• Clarify that fenced-code text is not commentable and add a user-facing changelog entry.

Validation

• npm test — 10 files and 130 tests passed
• Markdown diff and nested-fence structure checked

Manual QA

• Render the X Article reference → the comprehensive outer Markdown fence contains the inner Python fence without breaking
• Follow the documented code-block and comment guidance against a deployed Typefully environment → draft creation succeeds and code-only comment anchoring is avoided

After Merge

• Merge after typefully-backend#1882 and typefully-frontend#4001 are deployed

Related Links

• Backend contract: typefully-backend#1882
• Frontend support: typefully-frontend#4001
• Linear issue TYP-7441