Add multi-model panel with Fusion mode

Panel agents answer the user prompt in parallel (read-only, non-streaming);
their responses are merged into the worker's last user message as a
<panel_responses> block, and the session agent synthesizes one final
streamed answer. The RunPanel engine is built layer-ready for future MoA.

- New session fields: multi_model_mode (off/fusion), panel_agent_ids (CSV)
  with idempotent column migration
- New partial Services/AnthropicService.Panel.cs holding the RunPanel engine
- Provider-agnostic members: Anthropic RunNonStreamingToolLoop / OpenAI
  CompleteText / CLI plan-mode; all forced read-only, persisted with Tag=panel
- SSE events: panel_start, panel_member_start, panel_member_done, panel_done
- Frontend: multi-agent picker, live panel bubbles, history rendering
- Graceful degradation: panel errors fall back to worker-only

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/css/chat.css

@@ -1219,6 +1219,14 @@
   border-color: var(--info, #17a2b8);
 }
 
+.panel-bubble .review-header {
+  color: var(--accent, #9b6dff);
+}
+
+.panel-bubble {
+  border-color: var(--accent, #9b6dff);
+}
+
 .review-inline-feedback {
   color: var(--warning);
   margin-bottom: .4em;