Two optional, submitter-controlled fields on a catalog entry feed the
entry's own page at /docs/plugins/<name>:
- `screenshots:` — up to 6 https URLs on GitHub hosts (same host rule as
`image`, so the site never fetches from third-party hosts and a raw URL
pinned to the sha is as immutable as the code).
- `readme: true` — the docs build renders the README from the PINNED
commit (raw.githubusercontent.com / gitlab.com raw at <sha>), never live
content, so what a user reads is what the reviewer read.
Validator rejects malformed values (admission), the loader parses and
drops off-host screenshots with a warning (client), and the extractor
emits `screenshots`, `readme`, `readmeUrl` and a `maintainerSlug` for the
author pages. Tests on all three.
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