fix(installer): retry stashing app/worker through transient file locks on update

The update pipeline moved app/ and worker/ to .bak before extraction with no
retry, so a just-killed worker whose file handles had not been released yet
(WaitForExit returns before the OS flushes them) caused Directory.Move to throw
ERROR_SHARING_VIOLATION, stopping the pipeline on a cryptic error screen.

- DownloadAndExtractStep: stash/rollback moves+deletes now retry through
  transient IO/access errors (~5s); a persistent lock returns an actionable
  message instead of the raw error.
- StopWorkerStep: reading MainModule no longer skips Kill on failure, and a
  short settle follows the kill so handles are released before extraction.

src/ClaudeDo.Installer/Steps/DownloadAndExtractStep.cs

@@ -1,5 +1,6 @@
 using System.IO;
 using System.IO.Compression;
+using System.Threading;
 using ClaudeDo.Installer.Core;
 using ClaudeDo.Releases;
 
@@ -77,10 +78,23 @@ public sealed class DownloadAndExtractStep : IInstallStep
             var appBak = appDest + ".bak";
             var workerBak = workerDest + ".bak";
 
-            if (Directory.Exists(appBak))    Directory.Delete(appBak, recursive: true);
-            if (Directory.Exists(workerBak)) Directory.Delete(workerBak, recursive: true);
-            if (Directory.Exists(appDest))   Directory.Move(appDest, appBak);
-            if (Directory.Exists(workerDest)) Directory.Move(workerDest, workerBak);
+            try
+            {
+                if (Directory.Exists(appBak))    DeleteWithRetry(appBak);
+                if (Directory.Exists(workerBak)) DeleteWithRetry(workerBak);
+                if (Directory.Exists(appDest))   MoveWithRetry(appDest, appBak);
+                if (Directory.Exists(workerDest)) MoveWithRetry(workerDest, workerBak);
+            }
+            catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
+            {
+                // A just-stopped app/worker (or an Explorer/terminal window sitting in
+                // the install dir) still held a handle. Surface an actionable message
+                // instead of the raw "process cannot access the file" error.
+                return StepResult.Fail(
+                    "Could not replace the existing app/worker files — they are still in use. " +
+                    "Make sure ClaudeDo is fully closed (app and worker) and no Explorer or " +
+                    $"terminal window is open inside the install folder, then run the update again. Details: {ex.Message}");
+            }
 
             progress.Report("Extracting...");
             Directory.CreateDirectory(ctx.InstallDirectory);
@@ -91,17 +105,17 @@ public sealed class DownloadAndExtractStep : IInstallStep
             catch (Exception ex)
             {
                 // Roll back to previous binaries.
-                if (Directory.Exists(appDest))   Directory.Delete(appDest, recursive: true);
-                if (Directory.Exists(workerDest)) Directory.Delete(workerDest, recursive: true);
-                if (Directory.Exists(appBak))    Directory.Move(appBak, appDest);
-                if (Directory.Exists(workerBak)) Directory.Move(workerBak, workerDest);
+                if (Directory.Exists(appDest))   DeleteWithRetry(appDest);
+                if (Directory.Exists(workerDest)) DeleteWithRetry(workerDest);
+                if (Directory.Exists(appBak))    MoveWithRetry(appBak, appDest);
+                if (Directory.Exists(workerBak)) MoveWithRetry(workerBak, workerDest);
                 return StepResult.Fail(
                     $"Extraction failed; previous binaries have been restored: {ex.Message}.");
             }
 
             // Success — drop stash.
-            if (Directory.Exists(appBak))    Directory.Delete(appBak, recursive: true);
-            if (Directory.Exists(workerBak)) Directory.Delete(workerBak, recursive: true);
+            if (Directory.Exists(appBak))    DeleteWithRetry(appBak);
+            if (Directory.Exists(workerBak)) DeleteWithRetry(workerBak);
 
             ctx.InstalledVersion = release.TagName.TrimStart('v', 'V');
             return StepResult.Ok();
@@ -111,4 +125,26 @@ public sealed class DownloadAndExtractStep : IInstallStep
             try { Directory.Delete(scratchDir, recursive: true); } catch { /* best effort */ }
         }
     }
+
+    private static void MoveWithRetry(string source, string dest)
+        => RetryIo(() => Directory.Move(source, dest));
+
+    private static void DeleteWithRetry(string dir)
+        => RetryIo(() => Directory.Delete(dir, recursive: true));
+
+    // WaitForExit returns before Windows releases a just-killed process's file
+    // handles, so the stash Move/Delete can briefly hit a sharing violation.
+    // Retry through transient IO/access errors (~5s) before letting it surface.
+    private static void RetryIo(Action action)
+    {
+        const int attempts = 10;
+        for (var i = 0; ; i++)
+        {
+            try { action(); return; }
+            catch (Exception ex) when (ex is IOException or UnauthorizedAccessException && i < attempts - 1)
+            {
+                Thread.Sleep(500);
+            }
+        }
+    }
 }

src/ClaudeDo.Installer/Steps/StopWorkerStep.cs

@@ -19,22 +19,33 @@ public sealed class StopWorkerStep : IInstallStep
     {
         progress.Report("Stopping ClaudeDo processes (if running)...");
         var installDir = ctx.InstallDirectory;
+        var killedAny = false;
         foreach (var name in ProcessNames)
         {
             foreach (var p in Process.GetProcessesByName(name))
             {
                 try
                 {
-                    var path = p.MainModule?.FileName;
+                    // Scope to THIS install when the module path is readable; if it
+                    // can't be read (access race / exiting process), fall through and
+                    // kill anyway — a survivor would lock the install dir during
+                    // extraction. Reading MainModule must not skip the Kill.
+                    string? path = null;
+                    try { path = p.MainModule?.FileName; } catch { /* unreadable — kill anyway */ }
                     if (path is not null && !IsUnder(path, installDir)) continue;
                     p.Kill(entireProcessTree: true);
                     p.WaitForExit(10000);
+                    killedAny = true;
                 }
                 catch { /* process may have exited or be inaccessible */ }
                 finally { p.Dispose(); }
             }
         }
-        await Task.CompletedTask;
+
+        // WaitForExit returns before the OS releases the process's file handles.
+        // Give it a moment so DownloadAndExtractStep's Directory.Move doesn't race
+        // a still-open handle. (That step also retries, this just avoids the churn.)
+        if (killedAny) await Task.Delay(1500, ct);
         return StepResult.Ok();
     }