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/AgentStateRepository.cs

@@ -0,0 +1,76 @@
+using ControlCenter.Api.Data;
+using ControlCenter.Api.Entities;
+using ControlCenter.Api.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace ControlCenter.Api.Repositories;
+
+/// <summary>
+/// EF Core implementation of the Agent State Repository.
+/// Maps between the persisted <see cref="Agent"/> entity and the
+/// read-oriented <see cref="AgentState"/> model.
+/// </summary>
+public class AgentStateRepository : IAgentStateRepository
+{
+    private readonly AppDbContext _db;
+
+    public AgentStateRepository(AppDbContext db)
+    {
+        _db = db;
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<AgentState>> GetAllAsync(CancellationToken ct = default)
+    {
+        var agents = await _db.Agents
+            .AsNoTracking()
+            .OrderByDescending(a => a.LastActivity)
+            .ToListAsync(ct);
+
+        return agents.Select(ToModel).ToList();
+    }
+
+    /// <inheritdoc />
+    public async Task<AgentState?> GetBySessionKeyAsync(string sessionKey, CancellationToken ct = default)
+    {
+        var agent = await _db.Agents
+            .AsNoTracking()
+            .FirstOrDefaultAsync(a => a.SessionKey == sessionKey, ct);
+
+        return agent is null ? null : ToModel(agent);
+    }
+
+    /// <inheritdoc />
+    public async Task<bool> UpdateStatusAsync(Guid id, string status, CancellationToken ct = default)
+    {
+        if (!Enum.TryParse<AgentStatus>(status, ignoreCase: true, out var parsedStatus))
+            return false;
+
+        var agent = await _db.Agents.FindAsync([id], ct);
+        if (agent is null)
+            return false;
+
+        agent.Status = parsedStatus;
+        agent.UpdatedAt = DateTime.UtcNow;
+        agent.LastActivity = DateTime.UtcNow;
+
+        await _db.SaveChangesAsync(ct);
+        return true;
+    }
+
+    /// <summary>
+    /// Maps a persisted <see cref="Agent"/> entity to a <see cref="AgentState"/> model.
+    /// </summary>
+    private static AgentState ToModel(Agent agent) => new()
+    {
+        Id = agent.Id,
+        Status = agent.Status.ToString(),
+        Task = agent.Task,
+        Progress = agent.Progress,
+        SessionKey = agent.SessionKey,
+        Channel = agent.Channel,
+        LastActivity = agent.LastActivity,
+        CreatedAt = agent.CreatedAt,
+        UpdatedAt = agent.UpdatedAt,
+    };
+}

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);
+}