feat(typefully): support X Articles and reference guides

[frankdilo]

Add first-class X Article support to the Typefully skill and CLI while keeping articles inside the normal draft lifecycle, and split detailed Typefully guidance into on-demand reference guides.

Summary

• Add drafts:create and drafts:update support for --platform x_article with article-specific --content-markdown and --cover-media-id flags.
• Enforce X Article constraints: standalone platform only, no post-only flags, --all remains limited to post platforms, and --cover-media-id null removes the cover.
• Update comments:create so article comments do not require --post-index, while still rejecting non-zero article indexes.
• Move detailed article instructions into skills/typefully/references/platforms/x-articles.md, with payload examples, supported markdown, embeds, cover handling, and comment guidance.
• Split skill documentation into focused references for setup, comments, X, LinkedIn, X Articles, and local development while keeping SKILL.md concise.
• Document the dev-only --api-base-url override and cover it with a focused test.

Validation

• node --check skills/typefully/scripts/typefully.js
• npm run test:drafts
• npm run test:all
• git diff --check
• git diff --cached --check

Manual QA

• Create an X Article draft with markdown and a cover image.
• Update an X Article cover with --cover-media-id null.
• Create an X Article comment without --post-index.

After Merge

No action required.