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/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">