picker area should be big, and properly searchable

### Describe the bug

currently, when typing slash (/) e.g. a tiny 5 line picker area opens which cannot be searched. please provide fzf searchable list, with a 75% of page size picker hight.

### Affected version

_No response_

### Steps to reproduce the behavior

1. type /
2. type all reset
3. reset-allowed-tools is not found

if one knows the words and order, one can find with /re-al-to , which is not bad, but not really good.

### Expected behavior

_No response_

### Additional context

_No response_

Start at the slash-command picker opened by typing `/` and reproduce the five-line list behavior with `reset-allowed-tools`. Trace the picker implementation and its search handling; done means the picker uses an fzf-searchable list with roughly 75% page height and can find `reset-allowed-tools` without relying on abbreviated word order.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.