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/content/content.js

@@ -0,0 +1,80 @@
+// Content script for Cortex browser extension
+// Injected into web pages to extract content
+
+// Extract page content
+function extractContent() {
+  const title = document.title;
+
+  // Get selected text
+  const selection = window.getSelection()?.toString() || '';
+
+  // Try to find main content using common selectors
+  let content = '';
+  const contentSelectors = [
+    'article',
+    '[role="main"]',
+    'main',
+    '.post-content',
+    '.article-content',
+    '.entry-content',
+    '.content',
+    '#content',
+  ];
+
+  for (const selector of contentSelectors) {
+    const el = document.querySelector(selector);
+    if (el && el.textContent) {
+      content = el.textContent;
+      break;
+    }
+  }
+
+  // Fall back to body content
+  if (!content) {
+    content = document.body.textContent || '';
+  }
+
+  // Clean up content
+  content = content
+    .replace(/\s+/g, ' ')
+    .replace(/\n{3,}/g, '\n\n')
+    .trim()
+    .slice(0, 100000); // Limit to 100k chars
+
+  // Get meta description
+  const metaDesc = document.querySelector('meta[name="description"]')?.content || '';
+
+  // Get author
+  const author = document.querySelector('meta[name="author"]')?.content ||
+                 document.querySelector('[rel="author"]')?.textContent || '';
+
+  // Get published date
+  const datePublished = document.querySelector('meta[property="article:published_time"]')?.content ||
+                        document.querySelector('time[datetime]')?.getAttribute('datetime') || '';
+
+  return {
+    title,
+    content,
+    selection,
+    excerpt: metaDesc || content.slice(0, 200),
+    url: window.location.href,
+    author,
+    datePublished,
+  };
+}
+
+// Listen for messages from popup or background
+chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
+  if (request.action === 'extract') {
+    try {
+      const data = extractContent();
+      sendResponse(data);
+    } catch (err) {
+      sendResponse({ error: err.message });
+    }
+  }
+  return true; // Keep channel open for async response
+});
+
+// Make extract function available globally for background script injection
+window.__cortexExtract = extractContent;