Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

wwwroot/js/api.js

@@ -1,13 +1,48 @@
 // ── API Calls ──
 
-async function apiChat(model, messages, systemPrompt, sessionId) {
+async function apiChat(messages, systemPrompt, sessionId) {
   return fetch('/api/chat', {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
-    body: JSON.stringify({ model, messages, systemPrompt, sessionId })
+    body: JSON.stringify({ messages, systemPrompt, sessionId })
   });
 }
 
+// ── Agent API ──
+
+async function apiGetAgents() {
+  const resp = await fetch('/api/agents');
+  return resp.json();
+}
+
+async function apiGetAgent(agentId) {
+  const resp = await fetch(`/api/agents/${agentId}`);
+  if (!resp.ok) return null;
+  return resp.json();
+}
+
+async function apiCreateAgent(data) {
+  const resp = await fetch('/api/agents', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(data)
+  });
+  return resp.json();
+}
+
+async function apiUpdateAgent(agentId, data) {
+  const resp = await fetch(`/api/agents/${agentId}`, {
+    method: 'PUT',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(data)
+  });
+  return resp.json();
+}
+
+async function apiDeleteAgent(agentId) {
+  return fetch(`/api/agents/${agentId}`, { method: 'DELETE' });
+}
+
 async function apiHistory(sessionId) {
   const resp = await fetch(`/api/history?sessionId=${sessionId}`);
   return resp.json();