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>

Endpoints/ChatEndpoint.cs

@@ -230,6 +230,26 @@ public static class ChatEndpoint
 
         try
         {
+            // ── Multi-Modell-Panel (Fusion) ──
+            // Vor dem Worker: Panel-Agenten parallel antworten lassen und ihre Antworten dem
+            // Worker als Synthese-Block in die letzte User-Message einmergen. Der Worker streamt
+            // danach als Synthesizer über den unveränderten Dispatch unten. Heartbeat läuft bereits,
+            // Cancellation (workCt/sseCt) ist verfügbar. Nur bei aktivem Fusion-Modus + nicht-leerem
+            // Panel + keiner Choice-Antwort.
+            if (!isChoiceResponse && ctx.MultiModelMode == "fusion" && ctx.PanelAgents != null && ctx.PanelAgents.Count > 0)
+            {
+                try
+                {
+                    await anthropic.RunPanel(res, ctx.PanelAgents, messages, ctx.ChainId, sessionId, userApiKey?.ApiKey, workCt, sseCt);
+                }
+                catch (OperationCanceledException) { throw; }
+                catch (Exception ex)
+                {
+                    // Panel-Fehler darf den Worker nicht killen → ohne Synthese normal weiter.
+                    Console.WriteLine($"[Panel] Fehler, fahre ohne Synthese fort: {ex.Message}");
+                }
+            }
+
             if (!string.IsNullOrEmpty(ctx.Provider))
             {
                 // OpenAI-kompatibler Provider – Key des Users für diesen Agent verwenden