feat: gate sidebar chat behind --chat flag

$B connect (default): headed Chromium + extension with Activity + Refs
tabs only. No separate agent spawned. Clean, no confusion.

$B connect --chat: same + Chat tab with standalone claude -p agent.
Shows experimental banner: "Standalone mode — this is a separate
agent from your workspace."

Implementation:
- cli.ts: parse --chat, set BROWSE_SIDEBAR_CHAT env, conditionally
  spawn sidebar-agent
- server.ts: gate /sidebar-* routes behind chatEnabled, return 403
  when disabled, include chatEnabled in /health response
- sidepanel.js: applyChatEnabled() hides/shows Chat tab + banner
- background.js: forward chatEnabled from health response
- sidepanel.html/css: experimental banner with amber styling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/background.js

@@ -46,7 +46,8 @@ async function checkHealth() {
     if (data.status === 'healthy') {
       // Capture auth token from health response
       if (data.token) authToken = data.token;
-      setConnected(data);
+      // Forward chatEnabled so sidepanel can show/hide chat tab
+      setConnected({ ...data, chatEnabled: !!data.chatEnabled });
     } else {
       setDisconnected();
     }

extension/sidepanel.css

@@ -684,6 +684,19 @@ footer {
 }
 .port-input:focus { border-color: var(--amber-500); }
 
+/* ─── Experimental Banner ─────────────────────────────── */
+.experimental-banner {
+  background: rgba(245, 158, 11, 0.15);
+  border: 1px solid rgba(245, 158, 11, 0.3);
+  color: #F59E0B;
+  padding: 8px 12px;
+  border-radius: 6px;
+  font-size: 12px;
+  margin: 8px 12px;
+  text-align: center;
+  flex-shrink: 0;
+}
+
 /* ─── Accessibility ───────────────────────────────────── */
 :focus-visible {
   outline: 2px solid var(--amber-500);

extension/sidepanel.html

@@ -48,6 +48,11 @@
     <div class="refs-footer" id="refs-footer"></div>
   </main>
 
+  <!-- Experimental chat banner (shown when chatEnabled) -->
+  <div id="experimental-banner" class="experimental-banner" style="display: none;">
+    &#x26A0; Standalone mode &mdash; this is a separate agent from your workspace
+  </div>
+
   <!-- Command Bar -->
   <div class="command-bar">
     <input type="text" class="command-input" id="command-input" placeholder="Message Claude Code..." autocomplete="off" spellcheck="false">

extension/sidepanel.js

@@ -612,6 +612,7 @@ chrome.runtime.onMessage.addListener((msg) => {
     if (msg.data) {
       const url = `http://127.0.0.1:${msg.data.port || 34567}`;
       updateConnection(url, msg.data.token);
+      applyChatEnabled(!!msg.data.chatEnabled);
     } else {
       updateConnection(null);
     }
@@ -622,3 +623,39 @@ chrome.runtime.onMessage.addListener((msg) => {
     }
   }
 });
+
+// ─── Chat Gate ──────────────────────────────────────────────────
+// Show/hide Chat tab + command bar based on chatEnabled from server
+
+function applyChatEnabled(enabled) {
+  const commandBar = document.querySelector('.command-bar');
+  const chatTab = document.getElementById('tab-chat');
+  const banner = document.getElementById('experimental-banner');
+  const clearBtn = document.getElementById('clear-chat');
+
+  if (enabled) {
+    // Chat is enabled: show command bar, chat tab, experimental banner
+    if (commandBar) commandBar.style.display = '';
+    if (chatTab) chatTab.style.display = '';
+    if (banner) banner.style.display = '';
+    if (clearBtn) clearBtn.style.display = '';
+  } else {
+    // Chat disabled: hide command bar, chat content, clear button
+    if (commandBar) commandBar.style.display = 'none';
+    if (banner) banner.style.display = 'none';
+    if (clearBtn) clearBtn.style.display = 'none';
+    // If currently on chat tab, switch to activity
+    if (chatTab && chatTab.classList.contains('active')) {
+      chatTab.classList.remove('active');
+      // Open debug tabs and show activity
+      const debugToggle = document.getElementById('debug-toggle');
+      const debugTabs = document.getElementById('debug-tabs');
+      if (debugToggle) debugToggle.classList.add('active');
+      if (debugTabs) debugTabs.style.display = 'flex';
+      const activityTab = document.getElementById('tab-activity');
+      if (activityTab) activityTab.classList.add('active');
+      const activityBtn = document.querySelector('.tab[data-tab="activity"]');
+      if (activityBtn) activityBtn.classList.add('active');
+    }
+  }
+}