chore(worker): external MCP tools return explicit results instead of empty responses

set_task_config/set_list_config now echo which fields were set vs cleared, get_list_config/get_task_config return an explicit found=false instead of null, and delete_list/run_task_now/reset_failed_task/remove_task_attachment return a confirmation record — matching the found/ok convention already used by batch_get_tasks and get_task_log.

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -832,7 +832,8 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var result = await sut.GetTaskConfig(task.Id, CancellationToken.None);
 
-        Assert.Null(result);
+        Assert.False(result.Found);
+        Assert.Null(result.Config);
     }
 
     [Fact]
@@ -845,11 +846,12 @@ public sealed class ExternalMcpServiceTests : IDisposable
 
         var result = await sut.GetTaskConfig(task.Id, CancellationToken.None);
 
-        Assert.NotNull(result);
-        Assert.Equal("claude-sonnet-4-6", result.Model);
-        Assert.Equal("be concise", result.SystemPrompt);
-        Assert.Null(result.AgentPath);
-        Assert.Equal(10, result.MaxTurns);
+        Assert.True(result.Found);
+        Assert.NotNull(result.Config);
+        Assert.Equal("claude-sonnet-4-6", result.Config!.Model);
+        Assert.Equal("be concise", result.Config.SystemPrompt);
+        Assert.Null(result.Config.AgentPath);
+        Assert.Equal(10, result.Config.MaxTurns);
     }
 
     // ── GetTaskStatusValues ───────────────────────────────────────────────────