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>

Models/SessionRecord.cs

@@ -23,6 +23,8 @@ public class SessionRecord
     public int MaxReviewIterations { get; set; } = 3;
     public string FinalizerAgentId { get; set; }
     public string PreparerAgentId { get; set; }
+    public string MultiModelMode { get; set; }   // "off" / "fusion" (später "moa")
+    public string PanelAgentIds { get; set; }     // CSV der Panel-Agenten (Proposer)
     public string UnityMcpUrl { get; set; }
     public string UnityMcpToken { get; set; }
     public string CreatedAt { get; set; }