Adds ListPosts(ctx, ListPostsParams) to the content.Content capability so
wasm plugins can list posts with bodies — the per-plugin Postgres role
denies direct public.blog_posts reads (42501) and GetPost only fetches a
single post's metadata. Extends PostInfo with Body, AuthorName, AuthorSlug
and PublishedAt (additive; existing get_post golden unchanged). Wires all
layers: interface, ABI PostInfo/ContentListPosts{Request,Response} messages
(buf breaking clean), guest stub, and a deterministic content_list_posts
golden replayed by the cms host parity test.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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