Post Newsletter Natively

Convert a Substack newsletter post into a native posthog.com markdown file under contents/newsletter/.

The user will provide a Substack URL: $ARGUMENTS

Step 1: Fetch and reference

Run these in parallel:

Fetch the newsletter content from Substack's API, not with WebFetch. WebFetch routes the page through a summarizing model that will usually refuse to reproduce a full article verbatim ("I can't reproduce the full text verbatim…") and hand back a summary instead — useless when the whole job is copying text exactly. Substack exposes the post as JSON with no auth:
```
curl -sL "https://newsletter.posthog.com/api/v1/posts/{substack-slug}" -o /tmp/post.json
```
The response includes title, subtitle, post_date, cover_image, and body_html — the article body as HTML. Write body_html to a file and convert it to markdown with a script (unwrap Substack's <span> soup, turn <figure> blocks into image markers carrying both the src and the <figcaption>, and convert <a> to [text](href)). This gives you exact text, exact links, and image positions in one pass, and it doubles as the ground truth for Step 5.

Keep the JSON and the converted markdown around — Step 3b and Step 5 both reuse them.