Remove tool_choice agent option - auto-detect based on workspace/skills/memory

- Remove tool_choice from AgentEndpoint (create, update, list responses)
- ChatService: automatically set tool_choice='any' when workspace, memory
  or skills present, null otherwise. Same for preparer/review/finalizer
- SystemPromptBuilder: remove 3 toolAny-conditional prompt blocks
  (KRITISCH, Verhalten, ERINNERUNG)
- Frontend: remove checkbox, badge and JS references
- Agent.ToolChoice DB field preserved for backward compat, no longer read

wwwroot/index.html

@@ -198,10 +198,6 @@
           <div hidden>
             <label>Beschreibung</label>
             <input type="text" id="agentDescription" placeholder="Kurze Beschreibung (optional)">
-            <label class="checkbox-label">
-              <input type="checkbox" id="agentToolChoice">
-              Tool-Nutzung erzwingen <span class="checkbox-hint">(tool_choice: any)</span>
-            </label>
             <label class="checkbox-label">
               <input type="checkbox" id="agentAllowUserChoice" checked>
               Rückfragen erlauben <span class="checkbox-hint">(ask_user_choice Tool)</span>