feat(CUB-19): implement AgentStatus SignalR hub for real-time updates

- Add AgentStatusHub with typed IAgentStatusClient interface
  - Hub at /hubs/agent-status (matches design spec)
  - Fleet group + per-agent group subscription
  - AgentStatusChanged and AgentTaskProgress push events
  - Extension methods for server-side push via IHubContext

- Add GatewayEventBridgeService background service
  - Connects to OpenClaw Gateway WebSocket (v3 protocol)
  - Handles challenge → connect → hello-ok handshake
  - Bridges sessions.changed, session.message, session.tool events
  - Translates Gateway session status to AgentStatus enum
  - Maintains in-memory fleet state for snapshot queries

- Add REST API controllers
  - GET /api/agents — fleet status snapshot
  - GET /api/agents/{agentId} — single agent status
  - GET /api/logs/{agentId} — agent session logs (stub)
  - POST /api/command/stop/{agentId} — stop agent
  - POST /api/command/restart/{agentId} — restart agent
  - POST /api/command/steer/{agentId} — inject message

- Add models matching TypeScript spec interfaces
  - AgentStatusUpdate, TaskProgressUpdate, AgentCardData
  - AgentStatus enum (active/idle/thinking/error)

- Configure CORS with credentials for SignalR WebSocket
- Configure Swagger/OpenAPI with XML doc comments
- Agent role map matching frontend AGENT_ROLES constant

backend/ControlCenter/Controllers/AgentsController.cs

@@ -0,0 +1,71 @@
+using Microsoft.AspNetCore.Mvc;
+using ControlCenter.Services;
+
+namespace ControlCenter.Controllers;
+
+/// <summary>
+/// REST API for querying agent fleet status.
+/// Provides the initial data load for the Command Hub,
+/// while real-time updates flow through the AgentStatus SignalR hub.
+///
+/// <para>API contract for Rex (Frontend):</para>
+/// <list type="bullet">
+///   <item><c>GET /api/agents</c> — Returns all known agents with current status</item>
+///   <item><c>GET /api/agents/{agentId}</c> — Returns a specific agent's status</item>
+/// </list>
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+public class AgentsController : ControllerBase
+{
+    private readonly ILogger<AgentsController> _logger;
+    private readonly GatewayEventBridgeService _bridgeService;
+
+    public AgentsController(
+        ILogger<AgentsController> logger,
+        GatewayEventBridgeService bridgeService)
+    {
+        _logger = logger;
+        _bridgeService = bridgeService;
+    }
+
+    /// <summary>
+    /// Gets the current fleet status — all known agents with their latest state.
+    /// This is the initial load endpoint; subsequent updates arrive via SignalR.
+    /// </summary>
+    /// <returns>An array of agent card data for the entire fleet.</returns>
+    /// <response code="200">Returns the fleet snapshot.</response>
+    [HttpGet]
+    [ProducesResponseType(typeof(AgentCardData[]), StatusCodes.Status200OK)]
+    public IActionResult GetAgents()
+    {
+        var snapshot = _bridgeService.GetFleetSnapshot();
+        _logger.LogDebug("Fleet snapshot requested: {Count} agents", snapshot.Length);
+        return Ok(snapshot);
+    }
+
+    /// <summary>
+    /// Gets the current status of a specific agent.
+    /// </summary>
+    /// <param name="agentId">The agent identifier, e.g. "otto", "dex".</param>
+    /// <returns>The agent's current card data.</returns>
+    /// <response code="200">Returns the agent's status.</response>
+    /// <response code="404">Agent not found in the fleet state.</response>
+    [HttpGet("{agentId}")]
+    [ProducesResponseType(typeof(AgentCardData), StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    public IActionResult GetAgent(string agentId)
+    {
+        var snapshot = _bridgeService.GetFleetSnapshot();
+        var agent = snapshot.FirstOrDefault(a =>
+            a.Id.Equals(agentId, StringComparison.OrdinalIgnoreCase));
+
+        if (agent is null)
+        {
+            _logger.LogWarning("Agent not found: {AgentId}", agentId);
+            return NotFound(new { error = $"Agent '{agentId}' not found" });
+        }
+
+        return Ok(agent);
+    }
+}