Add an opt-in context menu for files and directories

Apps set browser.contextMenu to a list of { label, action, when?, danger? }
items; when(entry, path) scopes an item to directories, files, or any
other match. Right-click, the Menu key, or a touch long press opens it.
The demo adds sample items, including a .json-only one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

demo/index.html

@@ -6,12 +6,13 @@
     <meta name="color-scheme" content="light">
     <meta name="theme-color" content="#ffffff">
     <title>octopus-file-browser</title>
-    <script type="module" crossorigin src="/assets/index-CRttpLjx.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-rQJBLWWV.css">
+    <script type="module" crossorigin src="/assets/index-BJSID0yX.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-Bx92WdqS.css">
   </head>
   <body>
     <main>
       <octopus-file-browser></octopus-file-browser>
+      <p class="demo-status" role="status">Right-click or long-press a file or folder.</p>
     </main>
   </body>
 </html>