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

archetypes/post.md

@@ -0,0 +1,11 @@
+---
+title: "{{ replace (replaceRE `^\d{4}-\d{2}-\d{2}-` "" .File.ContentBaseName) "-" " " | title }}"
+author:
+# banner: image shown on the home page and at the top of the post, e.g. /media/posts/my-image.jpg
+# (file stored under static/media/)
+banner:
+# summary: optional; without it the text before <!--more--> (or the first lines) is used
+draft: true
+---
+
+Write the post here.