Items written in Markdown show formatted (#284)

A description with Markdown and no HTML tag is rendered to HTML with
pulldown-cmark, and an item with no text but RSS's <source:markdown>
(https://source.scripting.com/, as Scripting News sends beside its HTML) uses
that. HTML is left alone, and plain text without Markdown in it stays as it
was, so a line like "1984. ..." is not made a list. ammonia cleans the result
as any feed's HTML. Stored items keep what they have: the scanner does not
rewrite an item it has.

Reference: http://scripting.com/2026/10/06/005559.html

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/feed/Cargo.toml

@@ -9,6 +9,7 @@ anyhow.workspace = true
 atom_syndication.workspace = true
 chrono.workspace = true
 opml.workspace = true
+pulldown-cmark.workspace = true
 quick-xml.workspace = true
 reqwest.workspace = true
 rss.workspace = true