Add summary elements to Atom feeds

Should improve article previews in some readers. This also entailed
swapping out angle-brackets in one description to avoid errors.

content/posts/spoiler-element.md

@@ -1,6 +1,6 @@
 ---
 title: "Proposal: an HTML element for spoilers"
-description: "An informal proposal for dedicated elements for spoiler tags in HTML: use-cases, syntax, semantics, recommended UA behavior, and comparisons with <details>."
+description: "An informal proposal for dedicated elements for spoiler tags in HTML: use-cases, syntax, semantics, recommended UA behavior, and comparisons with “details”"
 date: 2023-11-12T13:48:00-08:00
 outputs:
     - html

layouts/_default/list.atom.xml

@@ -83,6 +83,9 @@
 		{{- $image = resources.Get (printf "/p/%s" . ) | resources.Fingerprint "md5" -}}
 		{{- end -}}
 		<media:thumbnail url="{{ $image.Permalink }}" width="{{ $image.Width }}" height="{{ $image.Height }}" />
+		{{ with .Description -}}
+		<summary type="text">{{ . }}</summary>
+		{{- end }}
 		<content type="xhtml" xml:base="{{ .Permalink }}">
 			<div xmlns="http://www.w3.org/1999/xhtml">
 					{{- if .Params.replyURI -}}