init: scaffold OPENBUREAU site with shibui-derived theme

- Hugo site for openbureau.ch (Deutsch, i18n-ready for EN/IT)
- Theme themes/openbureau/ = local copy of shibui, customized via
  site-level layouts and assets to keep the theme reference clean
- Editorial typography stack: Newsreader serif body, Space Grotesk
  display, Inter for listings, IBM Plex Mono for technical meta
- Content structure: library/ (Theorie, Büroführung, Software) with
  manifest and colophon at root; software is a library category, not
  a separate top-level
- Three views over one source: Journal (chronological home),
  Library (atlas grouped by section + tag cloud), single articles

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

themes/openbureau/layouts/shortcodes/mermaid.html

@@ -0,0 +1,2 @@
+<script src="https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js"></script>
+<div class="mermaid" align="{{ if .Get "align" }}{{ .Get "align" }}{{ else }}center{{ end }}">{{ safeHTML .Inner }}</div>