feat(ui): gate OperationTiming.Shared behind a kill switch, off by default

Its job (surfacing the DetailsIsland.BindAsync churn) is done; leave the
InvokeTimedAsync wrappers and Island Record call sites in place as a
chokepoint for next time, but stop writing by default. Shared now only
writes when CLAUDEDO_OP_TIMING=1 is set; the constructor keeps enabled=true
so both TestSetup redirects and OperationTimingTests are unaffected.

tests/ClaudeDo.Ui.Tests/Services/OperationTimingTests.cs

@@ -96,6 +96,27 @@ public class OperationTimingTests
         }
     }
 
+    [Fact]
+    public void Record_DoesNothingWhenDisabled()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "claudedo_optiming_" + Guid.NewGuid().ToString("N"));
+        var path = Path.Combine(dir, "operation-timing.ndjson");
+        var sink = new OperationTiming(path, enabled: false);
+
+        try
+        {
+            var exception = Record.Exception(() => sink.Record("hub", "RunNow", TimeSpan.FromMilliseconds(1), ok: false));
+
+            Assert.Null(exception);
+            Assert.False(File.Exists(path));
+            Assert.False(Directory.Exists(dir));
+        }
+        finally
+        {
+            if (Directory.Exists(dir)) Directory.Delete(dir, recursive: true);
+        }
+    }
+
     [Fact]
     public void Record_SwallowsWriteFailureInsteadOfThrowing()
     {