fix(installer): wrap WriteInstallManifestStep I/O in try/catch like sibling steps

src/ClaudeDo.Installer/Steps/WriteInstallManifestStep.cs

@@ -12,8 +12,10 @@ public sealed class WriteInstallManifestStep : IInstallStep
         if (string.IsNullOrWhiteSpace(ctx.InstalledVersion))
             return Task.FromResult(StepResult.Fail("Installed version is not set — DownloadAndExtractStep must run first."));
 
+        try
+        {
             var manifest = new InstallManifest(
-            Version: ctx.InstalledVersion!,
+                Version: ctx.InstalledVersion,
                 InstallDir: ctx.InstallDirectory,
                 WorkerDir: Path.Combine(ctx.InstallDirectory, "worker"),
                 InstalledAt: DateTimeOffset.UtcNow);
@@ -22,4 +24,9 @@ public sealed class WriteInstallManifestStep : IInstallStep
             progress.Report($"Wrote {InstallManifestStore.ManifestPath(ctx.InstallDirectory)}");
             return Task.FromResult(StepResult.Ok());
         }
+        catch (Exception ex)
+        {
+            return Task.FromResult(StepResult.Fail(ex.Message));
+        }
+    }
 }