[Feature]: Input box should support adding multiple skills continuously without re-typing /

## Description
Improve the skill selection experience in the input box when typing `/` to trigger the skill dropdown menu. Allow users to add multiple skills continuously without re-typing `/`.

## Problem Description (中文描述)
**问题：** 在输入框中输入 `/` 键时，会显示技能列表，但每次只能选择一个技能。添加一个技能后需要重新输入 `/` 才能添加下一个技能，这在使用多个技能时非常不便。

**Current Behavior:**
- When typing `/` in the input box, the skill dropdown menu appears
- Users can only add one skill at a time
- After adding a skill, users must manually type `/` again to add another skill
- This creates a cumbersome workflow when users want to use multiple skills in a single message

**Expected Behavior:**
- Support continuous selection of multiple skills from the dropdown
- Allow clicking/add multiple skills consecutively without re-typing `/`
- Show already-selected skills in the input box (e.g., as tags or highlighted text)

## Proposed Solution

### Option 1: Keep Dropdown Open After Selection (Recommended)
- After clicking a skill, keep the dropdown menu open
- Allow immediate selection of another skill
- Add visual indicator for selected skills in the input field

### Option 2: Multi-Select Mode
- Add checkboxes or multi-select capability in the dropdown
- Allow users to select multiple skills before confirming
- Single confirmation action adds all selected skills

### Option 3: Tag-based Input
- Skills appear as removable tags in the input box
- Type `/` to add more skills
- Click × on tags to remove them

## Example Workflow