fix: Plan-Modus – execute-Tool als readonly erlauben + Plaintext-Endpoint planMode-Parameter

- ToolService: execute in ReadOnlyToolNames aufgenommen
- SystemPromptBuilder: Plan-Modus Tools-Text um execute ergänzt
- SessionEndpoint: HandleSystemPrompt liest planMode Query-Parameter

Services/ToolService.cs

@@ -23,7 +23,7 @@ public class ToolService
     /// </summary>
     private static readonly HashSet<string> ReadOnlyToolNames = new HashSet<string>
     {
-        "read_file", "list_files", "glob", "grep", "todo_write",
+        "read_file", "list_files", "glob", "grep", "execute", "todo_write",
         "memory_read", "memory_write", "memory_list",
         AskUserChoiceToolName
     };