CUB-54: implement AgentState entity, repository, and DI registration

- Add AgentState entity mapping to 'agents' table (snake_case columns)
- Add IAgentStateRepository interface (GetAllAsync, GetBySessionKeyAsync, UpdateStatusAsync)
- Add AgentStateRepository with EF Core implementation
- Add ControlCenterDbContext with ApplyConfigurationsFromAssembly
- Add AgentStateConfiguration with snake_case column mappings and indexes
- Register DbContext (Npgsql) and repository in Program.cs DI
- Add ConnectionStrings to appsettings.json
- Add EF Core 9.0.7 and Npgsql.EntityFrameworkCore.PostgreSQL 9.0.4 packages

backend/ControlCenter/Models/AgentState.cs

@@ -0,0 +1,59 @@
+namespace ControlCenter.Models;
+
+/// <summary>
+/// Persistent state for an agent in the Control Center.
+/// Maps to the <c>agents</c> table in PostgreSQL.
+///
+/// <para>Tracks the operational status, current task, progress,
+/// session identity, and last activity timestamp for each agent
+/// in the minion fleet.</para>
+///
+/// <para>The <see cref="Status"/> property uses string values matching
+/// the AgentStatus enum: "active", "idle", "thinking", "error".
+/// Stored as <c>varchar</c> rather than a DB enum for schema flexibility.</para>
+/// </summary>
+public class AgentState
+{
+    /// <summary>
+    /// Unique identifier for the agent state record.
+    /// Defaults to a new <see cref="Guid"/> on creation.
+    /// </summary>
+    public Guid Id { get; set; } = Guid.NewGuid();
+
+    /// <summary>
+    /// Operational status of the agent.
+    /// Valid values: "active", "idle", "thinking", "error".
+    /// Maps to the AgentStatus enum used by SignalR.
+    /// </summary>
+    public string Status { get; set; } = "idle";
+
+    /// <summary>
+    /// Description of the agent's current task, if any.
+    /// Null when the agent is idle with no active assignment.
+    /// </summary>
+    public string? Task { get; set; }
+
+    /// <summary>
+    /// Task progress percentage (0–100).
+    /// Null when progress is not trackable for the current task.
+    /// </summary>
+    public int? Progress { get; set; }
+
+    /// <summary>
+    /// Full session key identifying the agent's active session.
+    /// Format: <c>agent:{agentId}:{channel}:...</c>
+    /// Used to correlate SignalR events with persistent state.
+    /// </summary>
+    public string SessionKey { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The channel the agent is operating on (e.g., "telegram", "discord", "slack").
+    /// </summary>
+    public string Channel { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Timestamp of the agent's last recorded activity.
+    /// Updated on every status change or task progress event.
+    /// </summary>
+    public DateTime LastActivity { get; set; } = DateTime.UtcNow;
+}