Add news posts with banner summaries on the home page

Posts live in content/post/; the five latest are summarised on the home
page. Builds for check/deploy go to build/ so they do not collide with a
running hugo server writing to public/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

layouts/post/section.html

@@ -0,0 +1,7 @@
+{{ define "main" }}
+<h1>{{ .Title }}</h1>
+<div class="content">{{ .Content }}</div>
+<div class="posts">
+  {{ range .RegularPages.ByDate.Reverse }}{{ partial "post-summary.html" . }}{{ end }}
+</div>
+{{ end }}