test(worker): isolate GitRepoFixture repos under a unique parent so sibling worktree paths never collide

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/Infrastructure/GitRepoFixture.cs

@@ -4,12 +4,16 @@ namespace ClaudeDo.Worker.Tests.Infrastructure;
 
 public sealed class GitRepoFixture : IDisposable
 {
+    private readonly string _rootDir;
+
     public string RepoDir { get; }
     public string BaseCommit { get; }
 
     public GitRepoFixture()
     {
-        RepoDir = Path.Combine(Path.GetTempPath(), $"claudedo_gittest_{Guid.NewGuid():N}");
+        // Repo one level below a unique parent: the sibling worktree root (.claudedo-worktrees) is the repo's parent, so it must be unique per fixture.
+        _rootDir = Path.Combine(Path.GetTempPath(), $"claudedo_gittest_{Guid.NewGuid():N}");
+        RepoDir = Path.Combine(_rootDir, "repo");
         Directory.CreateDirectory(RepoDir);
 
         RunGit(RepoDir, "init");
@@ -28,7 +32,7 @@ public sealed class GitRepoFixture : IDisposable
         try
         {
             // Force-remove read-only .git objects on Windows.
-            ForceDeleteDirectory(RepoDir);
+            ForceDeleteDirectory(_rootDir);
         }
         catch { /* best effort */ }
     }