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.

src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -5,6 +5,7 @@ using ClaudeDo.Data.Git;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
 using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Git;
 using ClaudeDo.Worker.State;
 using Microsoft.EntityFrameworkCore;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
@@ -366,9 +367,11 @@ public sealed class PlanningSessionManager
 
         var raw = strategy.Equals("central", StringComparison.OrdinalIgnoreCase)
             ? Path.Combine(centralRoot, "planning", taskId)
-            : Path.Combine(Path.GetDirectoryName(listWorkingDir)!, ".claudedo-worktrees", "planning", taskId);
+            : Path.Combine(WorktreeRootResolver.ResolveSiblingRoot(listWorkingDir), ".claudedo-worktrees", "planning", taskId);
 
-        return Path.GetFullPath(raw);
+        var worktreePath = Path.GetFullPath(raw);
+        WorktreeRootResolver.EnsureOutsideWorkingDir(worktreePath, listWorkingDir);
+        return worktreePath;
     }
 
     private static string TokenFilePathFor(string sessionDir) =>