Plugins in data/plugins are loaded at startup and can add a tab with a read-only node board, and the demo plugin shows every asset as a node (#113)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plugins/demo-plugin.js

@@ -0,0 +1,29 @@
+/**
+ * Demo plugin (#113): a tab "Assets" that shows every asset of the project as a node, laid out in
+ * a grid in the order of the asset list. It follows the project: adding, renaming or deleting an
+ * asset redraws the board.
+ *
+ * A plugin imports nothing – everything it needs comes in through `api`.
+ */
+
+export const meta = { id: 'demo', name: 'Demo plugin', version: '1.0.0', api: 1 };
+
+const COLUMNS = 4;
+const CELL_W = 220;
+const CELL_H = 64;
+
+export function activate(api) {
+  const board = new api.ui.NodeBoard();
+  const show = () =>
+    board.setNodes(
+      api.project.assets().map((asset, i) => ({
+        id: asset.id,
+        title: asset.name,
+        x: (i % COLUMNS) * CELL_W,
+        y: Math.floor(i / COLUMNS) * CELL_H,
+      })),
+    );
+  show();
+  api.project.on('change', show);
+  api.tabs.add({ id: 'assets', label: 'Assets', create: () => board });
+}