Add tests for Reddit structured artifacts

[EthanPlant]

Problem

Reddit is not just a text renderer. It produces a structured publishing draft with fields like subreddit, kind, title, URL, body, and suggested comment.

That structure is one of the reasons the Reddit renderer exists. It demonstrates that Elsewhere is not merely a platform-shaped text transformer.

The current test coverage should explicitly protect that behaviour.

Expected behaviour

A Reddit link post should produce a structured artifact containing:

subreddit
kind = link
title
url
comment

A Reddit self post should produce a structured artifact containing:

subreddit
kind = selfpost
title
body

Done when

• tests assert the artifact shape for Reddit link posts
• tests assert the artifact shape for Reddit self posts
• tests assert plan --json includes the Reddit artifact
• tests assert subreddit normalization is reflected in the artifact, not only in the preview text