Two fixes to the subagent progress display from PR #186:
1. Task index prefix: show 1-indexed prefix ([1], [2], ...) for ALL
tasks in batch mode (task_count > 1). Single tasks get no prefix.
Previously task 0 had no prefix while others did, making batch
output confusing.
2. Completion indicator: use spinner.print_above() instead of raw
print() for per-task completion lines (✓ [1/2] ...). Raw print
collided with the active spinner, mushing the completion text
onto the spinner line. Now prints cleanly above.
Added task_count parameter to _build_child_progress_callback and
_run_single_child. Updated tests accordingly.
41d8a80226