fix(worktrees): detect external merges, clean up empty dirs, prune in owning repo

- ContentInMain: HEAD ancestor of the default branch or squash-merged
  (merge-tree == target tree), against local origin/<default> without fetch;
  list_worktrees mergedIntoMain and the overview modal ("merged (external)")
  use it, "Cleanup finished" takes clean Active rows that are in main
- cleanup_task_worktree skips the dirty check for a non-git directory
- prune / worktree remove / branch -D run in the repo owning the worktree
  (git-common-dir), not the list's working dir; branch already gone reports
  branchDeleted=true

src/ClaudeDo.Worker/Hub/WorkerHub.cs

@@ -467,7 +467,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         return rows.Select(r => new WorktreeOverviewDto(
             r.TaskId, r.TaskTitle, r.TaskStatus, r.ListId, r.ListName,
             r.Path, r.BranchName, r.BaseCommit, r.State, r.DiffStat, r.CreatedAt, r.PathExistsOnDisk,
-            r.TaskNumber)).ToList();
+            r.TaskNumber, r.ContentInMain)).ToList();
     }
 
     public async Task<bool> SetWorktreeState(string taskId, WorktreeState newState)