feat(ui): Sentinel view — Yerin global security chat

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/components/sidebar.js

@@ -91,10 +91,10 @@ export function renderSidebar(root) {
     el('div', { class: 'sb-section' },
       el('div', { class: 'sb-title' }, 'Navigate'),
       navItem('Sacred Valley', '/sacred-valley'),
+      navItem('Sentinel', '/sentinel'),
       navItem('Search', '/search'),
       inboxItem,
       navItem('Jobs', '/jobs'),
-      el('div', { class: 'sb-item muted', title: 'Ships post-Plan-2' }, 'Agents — later'),
       el('div', { class: 'sb-item muted', title: 'Ships post-Plan-2' }, 'Resources — later')
     )
   );