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

Models/AgentRecord.cs

@@ -0,0 +1,16 @@
+namespace Gott.Models;
+
+public class AgentRecord
+{
+    public long Id { get; set; }
+    public string AgentId { get; set; }
+    public string Name { get; set; }
+    public string Avatar { get; set; }          // base64 data url
+    public string Description { get; set; }
+    public string Model { get; set; }           // sonnet, opus
+    public string Persona { get; set; }
+    public string Instructions { get; set; }
+    public bool IsDeleted { get; set; }
+    public string CreatedAt { get; set; }
+    public string UpdatedAt { get; set; }
+}