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/manifest.json

@@ -0,0 +1,36 @@
+{
+  "manifest_version": 3,
+  "name": "Cortex - Save to Memory",
+  "version": "1.0.0",
+  "description": "Save web content to your Cortex knowledge graph",
+  "permissions": [
+    "activeTab",
+    "contextMenus",
+    "storage"
+  ],
+  "host_permissions": [
+    "http://localhost:3100/*"
+  ],
+  "action": {
+    "default_popup": "popup/popup.html",
+    "default_icon": {
+      "16": "icons/icon-16.png",
+      "48": "icons/icon-48.png",
+      "128": "icons/icon-128.png"
+    }
+  },
+  "background": {
+    "service_worker": "background/background.js"
+  },
+  "content_scripts": [
+    {
+      "matches": ["<all_urls>"],
+      "js": ["content/content.js"]
+    }
+  ],
+  "icons": {
+    "16": "icons/icon-16.png",
+    "48": "icons/icon-48.png",
+    "128": "icons/icon-128.png"
+  }
+}