feat: Chrome extension Side Panel + Conductor API proposal

Chrome extension (Manifest V3, sideload):
- Side Panel with live activity feed, @ref overlays, dark terminal aesthetic
- Background worker: health polling, SSE relay, ref fetching
- Popup: port config, connection status, side panel launcher
- Content script: floating ref panel with @ref badges

Conductor API proposal (docs/designs/CONDUCTOR_SESSION_API.md):
- SSE endpoint for full Claude Code session mirroring in Side Panel
- Discovery via HTTP endpoint (not filesystem — extensions can't read files)

TODOS.md: add $B watch, multi-agent tabs, cross-platform CDP, Web Store publishing.
Mark CDP mode as shipped.

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

extension/manifest.json

@@ -0,0 +1,32 @@
+{
+  "manifest_version": 3,
+  "name": "gstack browse",
+  "version": "0.1.0",
+  "description": "Live activity feed and @ref overlays for gstack browse",
+  "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",
+      "128": "icons/icon-128.png"
+    }
+  },
+  "side_panel": {
+    "default_path": "sidepanel.html"
+  },
+  "background": {
+    "service_worker": "background.js"
+  },
+  "content_scripts": [{
+    "matches": ["<all_urls>"],
+    "js": ["content.js"],
+    "css": ["content.css"]
+  }],
+  "icons": {
+    "16": "icons/icon-16.png",
+    "48": "icons/icon-48.png",
+    "128": "icons/icon-128.png"
+  }
+}