feat(CUB-54): implement Agent State Repository with EF Core

- Add AgentState read model (Models/AgentState.cs)
- Add IAgentStateRepository interface with GetAllAsync,
  GetBySessionKeyAsync, UpdateStatusAsync
- Add AgentStateRepository EF Core implementation
  mapping Agent entity → AgentState model
- Register IAgentStateRepository in DI (Program.cs)
- Exclude ControlCenter sub-project from Api compilation

Build: 0 warnings, 0 errors

backend/Models/AgentState.cs

@@ -0,0 +1,19 @@
+namespace ControlCenter.Api.Models;
+
+/// <summary>
+/// Read-only model representing an agent's current state.
+/// Used as the return type from the Agent State Repository
+/// to decouple consumers from the persistence layer.
+/// </summary>
+public class AgentState
+{
+    public Guid Id { get; set; }
+    public string Status { get; set; } = string.Empty;
+    public string? Task { get; set; }
+    public int? Progress { get; set; }
+    public string SessionKey { get; set; } = string.Empty;
+    public string Channel { get; set; } = string.Empty;
+    public DateTime LastActivity { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime UpdatedAt { get; set; }
+}