feat(worker): add external MCP agent-listing tool

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/AgentMcpTools.cs

@@ -0,0 +1,18 @@
+using System.ComponentModel;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Worker.Agents;
+using ModelContextProtocol.Server;
+
+namespace ClaudeDo.Worker.External;
+
+[McpServerToolType]
+public sealed class AgentMcpTools
+{
+    private readonly AgentFileService _agents;
+
+    public AgentMcpTools(AgentFileService agents) => _agents = agents;
+
+    [McpServerTool, Description("List available agent definition files (name, description, path) for use as a task's agent path.")]
+    public async Task<IReadOnlyList<AgentInfo>> ListAgents(CancellationToken cancellationToken)
+        => await _agents.ScanAsync(cancellationToken);
+}