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.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);