Deleting multiple threads asks about each orphaned worktree in a separate dialog

Selecting several threads in the sidebar and choosing Delete confirms the batch once, then asks "delete the worktree too?" separately for every selected thread that has one. Worse, the deletion for the previous row runs before the next dialog appears, so clearing ten threads means answering the same question ten times with a couple of seconds of dead air in between.

Steps to reproduce

1. Create several threads, each with its own worktree.
2. Multi-select them in the sidebar and pick Delete.
3. Confirm the "Delete N threads" dialog.

Current behavior

Seven threads selected, each with its own worktree:

Then the same question, once per thread, with a deletion round trip in between:

Suggested behavior

Count the worktrees the whole selection would orphan, ask once, and apply that single answer to every deletion. A worktree should only count when every thread pointing at it is inside the selection. Single-thread deletes keep the existing per-thread prompt.

Fixed by #6594.

Review the bulk thread-deletion flow described in the issue and compare it with the fix in #6594. Verify that selected threads sharing orphaned worktrees produce one prompt, while single-thread deletion retains its existing prompt; confirm the behavior by reproducing the multi-select scenario.