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/content.css

@@ -0,0 +1,77 @@
+/* gstack browse — ref overlay styles */
+
+#gstack-ref-overlays {
+  font-family: 'SF Mono', 'Fira Code', monospace !important;
+}
+
+.gstack-ref-badge {
+  position: absolute;
+  background: rgba(220, 38, 38, 0.9);
+  color: #fff;
+  font-size: 10px;
+  font-weight: 700;
+  padding: 1px 4px;
+  border-radius: 3px;
+  line-height: 14px;
+  pointer-events: none;
+  z-index: 2147483647;
+}
+
+/* Floating ref panel (used when positions are unknown) */
+.gstack-ref-panel {
+  position: fixed;
+  bottom: 12px;
+  right: 12px;
+  width: 220px;
+  max-height: 300px;
+  background: rgba(10, 10, 10, 0.95);
+  border: 1px solid #333;
+  border-radius: 6px;
+  overflow: hidden;
+  pointer-events: auto;
+  box-shadow: 0 4px 24px rgba(0, 0, 0, 0.5);
+  font-size: 11px;
+}
+
+.gstack-ref-panel-header {
+  padding: 6px 10px;
+  background: #0f0f0f;
+  border-bottom: 1px solid #222;
+  color: #fff;
+  font-weight: 600;
+  font-size: 11px;
+}
+
+.gstack-ref-panel-list {
+  max-height: 260px;
+  overflow-y: auto;
+}
+
+.gstack-ref-panel-row {
+  padding: 3px 10px;
+  border-bottom: 1px solid #1a1a1a;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.gstack-ref-panel-id {
+  color: #4ade80;
+  font-weight: 600;
+  margin-right: 4px;
+}
+
+.gstack-ref-panel-role {
+  color: #888;
+  margin-right: 4px;
+}
+
+.gstack-ref-panel-name {
+  color: #e0e0e0;
+}
+
+.gstack-ref-panel-more {
+  padding: 4px 10px;
+  color: #666;
+  font-style: italic;
+}