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/ChatModels.cs

@@ -14,9 +14,6 @@ public class ChatMessage
 
 public class ChatRequest
 {
-    [JsonPropertyName("model")]
-    public string Model { get; set; } = "sonnet";
-
     [JsonPropertyName("messages")]
     public List<ChatMessage> Messages { get; set; } = new();