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/index.html

@@ -156,6 +156,19 @@
               <div class="agent-select-empty"><i class="fa fa-flag-checkered"></i> Kein Finalizer-Agent</div>
             </div>
             <input type="hidden" id="finalizerAgentSelect" value="">
+            <label>Multi-Modell-Panel <small>(Fusion)</small></label>
+            <select id="multiModelMode">
+              <option value="off">Aus</option>
+              <option value="fusion">Fusion – Panel antwortet parallel, Session-Agent synthetisiert</option>
+            </select>
+            <div id="panelAgentsRow" hidden>
+              <label>Panel-Agenten <small>(antworten parallel zur Anfrage)</small></label>
+              <div id="panelAgentsCard" class="agent-select-card">
+                <div class="agent-select-empty"><i class="fa fa-users"></i> Keine Panel-Agenten</div>
+              </div>
+              <input type="hidden" id="panelAgentIds" value="">
+              <small class="muted">Jeder Panel-Agent kostet zusätzlich Tokens/Zeit – alle laufen pro Nachricht. Member arbeiten read-only.</small>
+            </div>
             <label>Benutzerdefinierte Befehle</label>
             <div class="custom-commands" id="customCommandsList"></div>
             <button type="button" id="addCommandBtn" class="add-cmd-btn">+ Befehl hinzufügen</button>