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/background/background.js

@@ -0,0 +1,136 @@
+// Background service worker for Cortex browser extension
+
+const CORTEX_API = 'http://localhost:3100/api';
+
+// Create context menu on install
+chrome.runtime.onInstalled.addListener(() => {
+  // Save page context menu
+  chrome.contextMenus.create({
+    id: 'cortex-save-page',
+    title: 'Save page to Cortex',
+    contexts: ['page'],
+  });
+
+  // Save selection context menu
+  chrome.contextMenus.create({
+    id: 'cortex-save-selection',
+    title: 'Save selection to Cortex',
+    contexts: ['selection'],
+  });
+
+  // Save link context menu
+  chrome.contextMenus.create({
+    id: 'cortex-save-link',
+    title: 'Save link to Cortex',
+    contexts: ['link'],
+  });
+});
+
+// Handle context menu clicks
+chrome.contextMenus.onClicked.addListener(async (info, tab) => {
+  try {
+    let content = '';
+    let title = tab.title;
+
+    if (info.menuItemId === 'cortex-save-selection') {
+      content = info.selectionText || '';
+      title = `Selection from: ${tab.title}`;
+    } else if (info.menuItemId === 'cortex-save-link') {
+      content = `Link: ${info.linkUrl}`;
+      title = info.linkUrl;
+    } else {
+      // Get full page content
+      const [{ result }] = await chrome.scripting.executeScript({
+        target: { tabId: tab.id },
+        func: extractPageContent,
+      });
+      content = result?.content || '';
+      title = result?.title || tab.title;
+    }
+
+    // Save to Cortex
+    await saveToCortex({
+      title,
+      content,
+      url: tab.url,
+      kind: 'memory',
+      tags: ['web-clip'],
+    });
+
+    // Show notification (if supported)
+    if (chrome.notifications) {
+      chrome.notifications.create({
+        type: 'basic',
+        iconUrl: 'icons/icon-48.png',
+        title: 'Saved to Cortex',
+        message: title.slice(0, 50),
+      });
+    }
+  } catch (err) {
+    console.error('Context menu action failed:', err);
+  }
+});
+
+// Function to extract page content (injected into page)
+function extractPageContent() {
+  // Simple content extraction
+  // A full implementation would use Readability
+  const title = document.title;
+
+  // Try to get main content
+  const article = document.querySelector('article');
+  const main = document.querySelector('main');
+  const content = article?.textContent || main?.textContent || document.body.textContent || '';
+
+  // Clean up content
+  const cleanContent = content
+    .replace(/\s+/g, ' ')
+    .replace(/\n+/g, '\n')
+    .trim()
+    .slice(0, 50000); // Limit content size
+
+  return {
+    title,
+    content: cleanContent,
+    url: window.location.href,
+  };
+}
+
+// Save to Cortex API
+async function saveToCortex(data) {
+  const response = await fetch(`${CORTEX_API}/nodes`, {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+    },
+    body: JSON.stringify({
+      kind: data.kind || 'memory',
+      title: data.title,
+      content: data.content,
+      tags: data.tags || ['web-clip'],
+      metadata: {
+        source: {
+          type: 'url',
+          url: data.url,
+          savedAt: Date.now(),
+        },
+      },
+    }),
+  });
+
+  if (!response.ok) {
+    throw new Error(`HTTP ${response.status}`);
+  }
+
+  return response.json();
+}
+
+// Listen for messages from content script or popup
+chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
+  if (request.action === 'save') {
+    saveToCortex(request.data)
+      .then(node => sendResponse({ success: true, nodeId: node.id }))
+      .catch(err => sendResponse({ success: false, error: err.message }));
+    return true; // Keep channel open for async response
+  }
+});