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/assets/css/custom.css

@@ -0,0 +1,50 @@
+/* Quadratisches Bild */
+.list-image {
+  aspect-ratio: 9 / 5;
+  overflow: hidden;
+}
+
+.list-image img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  display: block;
+  margin: 0;
+}
+
+/* Titel + Meta in einer Zeile */
+.list-title-row {
+  display: flex;
+  justify-content: space-between;
+  align-items: baseline;
+  margin-top: 0.4rem;
+}
+
+.list-title {
+  margin: 0;
+}
+
+.list-meta {
+  display: flex;
+  gap: 8px;
+}
+
+/* Grayscale */
+img {
+  filter: grayscale(100%);
+  transition: filter 0.4s ease;
+}
+img:hover {
+  filter: grayscale(0%);
+}
+.auto-carousel img {
+  filter: none !important;
+}
+body {
+  padding: var(--spacing-sm) 1.5rem;
+  gap: var(--spacing-sm);
+}
+
+img {
+  margin: 0;
+}