System prompt: build from agent persona/instructions, store in chain, remove UI textarea

wwwroot/js/api.js

@@ -1,10 +1,10 @@
 // ── API Calls ──
 
-async function apiChat(messages, systemPrompt, sessionId) {
+async function apiChat(messages, sessionId) {
   return fetch('/api/chat', {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
-    body: JSON.stringify({ messages, systemPrompt, sessionId })
+    body: JSON.stringify({ messages, sessionId })
   });
 }