chore: remove dead MergeFfOnlyAsync, split AgentMcpTools into its own file, fix stale Zitadel doc

- GitService.MergeFfOnlyAsync had no callers anywhere in code; removed.
- AgentMcpTools moved out of LifecycleMcpTools.cs into External/AgentMcpTools.cs (no behavior change).
- Worker CLAUDE.md described ZitadelAuthProvider as a Phase 2 stub; it's fully implemented (OIDC discovery + refresh-token flow), updated both mentions.

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);
+}

src/ClaudeDo.Worker/External/LifecycleMcpTools.cs

@@ -1,25 +1,11 @@
 using System.ComponentModel;
-using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
-using ClaudeDo.Worker.Agents;
 using ClaudeDo.Worker.Lifecycle;
 using ModelContextProtocol.Server;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 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);
-}
-
 [McpServerToolType]
 public sealed class LifecycleMcpTools
 {