fix(worker): stop sibling worktrees from landing inside the repo tree

Path.GetDirectoryName returned workingDir itself instead of its parent
whenever list.WorkingDir ended in a directory separator, placing
.claudedo-worktrees inside the target repo's own working tree. Add
WorktreeRootResolver to normalize the trailing separator before deriving
the sibling root and to guard that the resulting worktree path never
lands inside workingDir, used by both WorktreeManager and
PlanningSessionManager. Add a startup sweep (LegacyWorktreeFolderRecovery)
that warns when a list's working dir already contains a leftover
.claudedo-worktrees folder from before the fix.

tests/ClaudeDo.Worker.Tests/Planning/PlanningSessionManagerTests.cs

@@ -123,6 +123,30 @@ public sealed class PlanningSessionManagerTests : IDisposable
         Assert.NotNull(loaded.PlanningSessionToken);
     }
 
+    [Fact]
+    public async Task StartAsync_TrailingSeparatorOnWorkingDir_PlacesWorktreeOutsideRepo()
+    {
+        var listId = Guid.NewGuid().ToString();
+        var wd = Path.Combine(Path.GetTempPath(), $"cd_wd_{Guid.NewGuid():N}");
+        GitRepoFixture.InitRepoWithInitialCommit(wd);
+        await _lists.AddAsync(new ListEntity
+        {
+            Id = listId,
+            Name = "Test",
+            WorkingDir = wd + Path.DirectorySeparatorChar,
+            CreatedAt = DateTime.UtcNow,
+        });
+        var parent = await SeedManualTaskAsync(listId);
+
+        var ctx = await _sut.StartAsync(parent.Id, CancellationToken.None);
+
+        var wdFull = Path.GetFullPath(wd);
+        Assert.False(
+            ctx.WorktreePath.StartsWith(wdFull + Path.DirectorySeparatorChar, StringComparison.OrdinalIgnoreCase),
+            $"worktree path {ctx.WorktreePath} should not be nested inside the repo dir {wdFull}");
+        Assert.True(Directory.Exists(ctx.WorktreePath));
+    }
+
     [Fact]
     public async Task StartAsync_TaskNotManual_Throws()
     {