Add browser extension and shell completions (Milestones 12-13)

M12: Browser Extension
- Chrome/Edge Manifest V3 extension
- Popup UI for saving pages
- Context menu integration (save page/selection/link)
- Background service worker
- Content script for extraction

M13: Shell Completions
- Bash, Zsh, Fish, PowerShell completions
- Dynamic node ID completion
- Dynamic tag completion
- Dynamic graph completion
- Auto-install command (--install)

extension/icons/icon.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#4fc3f7;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#29b6f6;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  <circle cx="64" cy="64" r="56" fill="url(#grad)"/>
+  <g fill="#fff">
+    <!-- Brain/memory icon representation -->
+    <circle cx="64" cy="48" r="8"/>
+    <circle cx="44" cy="68" r="6"/>
+    <circle cx="84" cy="68" r="6"/>
+    <circle cx="54" cy="88" r="5"/>
+    <circle cx="74" cy="88" r="5"/>
+    <!-- Connections -->
+    <path d="M64 56 L44 62" stroke="#fff" stroke-width="2" fill="none"/>
+    <path d="M64 56 L84 62" stroke="#fff" stroke-width="2" fill="none"/>
+    <path d="M44 74 L54 83" stroke="#fff" stroke-width="2" fill="none"/>
+    <path d="M84 74 L74 83" stroke="#fff" stroke-width="2" fill="none"/>
+    <path d="M54 88 L74 88" stroke="#fff" stroke-width="2" fill="none"/>
+  </g>
+</svg>