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

wwwroot/js/pages/agent-page.js

@@ -1,6 +1,7 @@
 // ── Agent Page ──
 
 let _agentAvatarEditor = null;
+let _agentTaskMde = null;
 
 registerPage('agent', {
   async init(params) {
@@ -44,6 +45,21 @@ registerPage('agent', {
       initialOffsetY: (agent && agent.avatarOffsetY) || 0
     });
 
+    // EasyMDE für Task-Instruction
+    const taskEl = document.getElementById('agentTaskInstruction');
+    _agentTaskMde = new EasyMDE({
+      element: taskEl,
+      toolbar: ['bold', 'italic', 'heading', '|', 'quote', 'unordered-list', 'ordered-list', '|', 'link', 'code', '|', 'preview', 'guide'],
+      status: false,
+      autofocus: false,
+      spellChecker: false,
+      autoDownloadFontAwesome: false,
+      placeholder: 'Spezifische Anweisungen für diesen Agent (Markdown, optional)',
+      minHeight: '120px'
+    });
+    _agentTaskMde.codemirror.setOption('theme', 'default');
+    _agentTaskMde.codemirror.getWrapperElement().closest('.EasyMDEContainer').classList.add('fe-mde-wrap');
+
     if (agent) {
       title.textContent = agent.name;
       nameEl.value = agent.name || '';
@@ -51,6 +67,7 @@ registerPage('agent', {
       modelEl.value = agent.model || 'sonnet';
       toolChoiceEl.checked = (agent.toolChoice || 'any') === 'any';
       allowUserChoiceEl.checked = agent.allowUserChoice !== false;
+      _agentTaskMde.value(agent.taskInstruction || '');
       autoCommitEl.value = agent.autoCommit || '';
       useSoulEl.checked = agent.useSoul !== false;
       useIdentityEl.checked = agent.useIdentity !== false;
@@ -107,7 +124,8 @@ registerPage('agent', {
         avatarScale: _agentAvatarEditor.getScale(),
         avatarOffsetX: _agentAvatarEditor.getOffsetX(),
         avatarOffsetY: _agentAvatarEditor.getOffsetY(),
-        customCommands: getCommandsFromUI(commandsList)
+        customCommands: getCommandsFromUI(commandsList),
+        taskInstruction: _agentTaskMde ? _agentTaskMde.value().trim() || null : null
       };
 
       if (isNew) {
@@ -137,6 +155,10 @@ registerPage('agent', {
     });
   },
   destroy() {
+    if (_agentTaskMde) {
+      try { _agentTaskMde.toTextArea(); } catch (_) {}
+      _agentTaskMde = null;
+    }
     if (_agentAvatarEditor) {
       _agentAvatarEditor.destroy();
       _agentAvatarEditor = null;