docs: document article mode in agent skill

[ryanfowler]

Summary

• add article extraction to the fetch skill description and common examples
• document Markdown/frontmatter behavior, final-URL link resolution, and the 16 MiB limit
• clarify raw output handling and JavaScript-rendered page limitations

Validation

• git diff --check

Caveat

• Prettier was unavailable locally; no Cargo tests were run for this docs-only change.