feat: sidebar chat with Claude Code — icon opens side panel directly

Replace popup flyout with direct side panel open on icon click. Primary
UI is now a chat interface that sends messages to Claude Code via file
queue. Activity/Refs tabs moved behind a debug toggle in the footer.
Command bar with history, auto-poll for responses, amber design system.

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

extension/manifest.json

@@ -6,7 +6,6 @@
   "permissions": ["sidePanel", "storage", "activeTab"],
   "host_permissions": ["http://127.0.0.1:*/"],
   "action": {
-    "default_popup": "popup.html",
     "default_icon": {
       "16": "icons/icon-16.png",
       "48": "icons/icon-48.png",