feat: Anweisungen-Feld für Agents mit EasyMDE-Editor

- AgentRecord: TaskInstruction (string, nullable)
- DB-Migration: task_instruction TEXT Spalte
- AgentEndpoint: Create/Update lesen taskInstruction
- SystemPromptBuilder: '## Anweisungen' Block nach Soul, nur wenn nicht leer
- HTML: Textarea #agentTaskInstruction nach Rückfragen-Checkbox
- agent-page.js: EasyMDE-Editor mit Init, Laden, Speichern, Destroy

Endpoints/AgentEndpoint.cs

@@ -66,6 +66,7 @@ public static class AgentEndpoint
             UseWebSearch = body.TryGetProperty("webSearchVersion", out var wsv) && !string.IsNullOrEmpty(wsv.GetString()),
             WebSearchVersion = body.TryGetProperty("webSearchVersion", out var wsv2) ? wsv2.GetString() ?? "" : "",
             AllowUserChoice = !body.TryGetProperty("allowUserChoice", out var auc) || auc.GetBoolean(),
+            TaskInstruction = body.TryGetProperty("taskInstruction", out var ti) ? ti.GetString() : null,
             AvatarScale = body.TryGetProperty("avatarScale", out var asc) ? asc.GetDouble() : 1.0,
             AvatarOffsetX = body.TryGetProperty("avatarOffsetX", out var aox) ? aox.GetDouble() : 0,
             AvatarOffsetY = body.TryGetProperty("avatarOffsetY", out var aoy) ? aoy.GetDouble() : 0
@@ -105,6 +106,7 @@ public static class AgentEndpoint
             agent.UseWebSearch = !string.IsNullOrEmpty(agent.WebSearchVersion);
         }
         if (body.TryGetProperty("allowUserChoice", out var auc)) agent.AllowUserChoice = auc.GetBoolean();
+        if (body.TryGetProperty("taskInstruction", out var ti)) agent.TaskInstruction = ti.GetString();
         if (body.TryGetProperty("avatarScale", out var asc)) agent.AvatarScale = asc.GetDouble();
         if (body.TryGetProperty("avatarOffsetX", out var aox)) agent.AvatarOffsetX = aox.GetDouble();
         if (body.TryGetProperty("avatarOffsetY", out var aoy)) agent.AvatarOffsetY = aoy.GetDouble();