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/Repositories/IAgentStateRepository.cs

@@ -0,0 +1,27 @@
+using ControlCenter.Api.Models;
+
+namespace ControlCenter.Api.Repositories;
+
+/// <summary>
+/// Repository interface for accessing and mutating Agent State.
+/// Provides a clean abstraction over the EF Core data access layer.
+/// </summary>
+public interface IAgentStateRepository
+{
+    /// <summary>
+    /// Retrieve all agent states.
+    /// </summary>
+    Task<IReadOnlyList<AgentState>> GetAllAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Retrieve a single agent state by its session key.
+    /// Returns null if no agent is found with the given session key.
+    /// </summary>
+    Task<AgentState?> GetBySessionKeyAsync(string sessionKey, CancellationToken ct = default);
+
+    /// <summary>
+    /// Update the status of an agent by its primary key.
+    /// Returns true if the agent was found and updated, false otherwise.
+    /// </summary>
+    Task<bool> UpdateStatusAsync(Guid id, string status, CancellationToken ct = default);
+}