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>

Services/ChatService.cs

@@ -143,6 +143,22 @@ public class ChatService
         var finalizerContext = await BuildSubAgentContext(
             session.FinalizerAgentId, sessionId, workspace, session, user);
 
+        // Multi-Modell-Panel (Fusion): Panel-Agenten (Proposer) bauen, falls aktiviert.
+        // Jeder Panel-Agent ist ein vollwertiger SubAgentContext (Modell/Provider/Key/Prompt/Effort).
+        var panelContexts = new List<SubAgentContext>();
+        if (!string.IsNullOrEmpty(session.MultiModelMode) && session.MultiModelMode != "off"
+            && !string.IsNullOrWhiteSpace(session.PanelAgentIds))
+        {
+            var panelIds = session.PanelAgentIds
+                .Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+                .Distinct();
+            foreach (var pid in panelIds)
+            {
+                var pc = await BuildSubAgentContext(pid, sessionId, workspace, session, user);
+                if (pc != null) panelContexts.Add(pc);
+            }
+        }
+
         if (preparerContext != null)
         {
             preparerContext.AllowUserChoice = preparerContext.Agent?.AllowUserChoice ?? false;
@@ -204,6 +220,8 @@ Für welche Plattform soll die App entwickelt werden? Web, Mobile oder Desktop?
             Preparer = preparerContext,
             Review = reviewContext,
             Finalizer = finalizerContext,
+            MultiModelMode = session.MultiModelMode,
+            PanelAgents = panelContexts,
             PlanMode = planMode
         };
     }