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/README.md

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+# Cortex Browser Extension
+
+Save web content directly to your Cortex knowledge graph.
+
+## Features
+
+- One-click save from any webpage
+- Right-click context menu integration
+- Save selected text only
+- Auto-extract page title and content
+- Tag suggestions based on URL
+
+## Installation
+
+### Chrome / Edge
+
+1. Open `chrome://extensions` (or `edge://extensions`)
+2. Enable "Developer mode" in the top right
+3. Click "Load unpacked"
+4. Select this `extension` folder
+
+### Firefox
+
+1. Open `about:debugging`
+2. Click "This Firefox"
+3. Click "Load Temporary Add-on"
+4. Select `manifest.json` from this folder
+
+## Usage
+
+### Popup
+
+1. Click the Cortex icon in your browser toolbar
+2. Edit the title and tags as needed
+3. Choose whether to include page content
+4. Click "Save"
+
+### Context Menu
+
+- **Save page**: Right-click anywhere on a page → "Save page to Cortex"
+- **Save selection**: Select text, right-click → "Save selection to Cortex"
+- **Save link**: Right-click a link → "Save link to Cortex"
+
+## Requirements
+
+The Cortex server must be running for the extension to work:
+
+```bash
+cortex serve
+```
+
+By default, the extension connects to `http://localhost:3100/api`.
+
+## Development
+
+The extension uses Manifest V3 and consists of:
+
+- `manifest.json` - Extension configuration
+- `popup/` - Popup UI (HTML, CSS, JS)
+- `background/` - Service worker for context menus
+- `content/` - Content script for page extraction
+- `icons/` - Extension icons
+
+## Icon Generation
+
+To generate PNG icons from the SVG:
+
+```bash
+# Using ImageMagick
+convert icons/icon.svg -resize 16x16 icons/icon-16.png
+convert icons/icon.svg -resize 48x48 icons/icon-48.png
+convert icons/icon.svg -resize 128x128 icons/icon-128.png
+```
+
+Or use an online SVG to PNG converter.