Distinguish "agent working" vs "awaiting user input" vs "turn complete" in the title-bar indicator

### Describe the feature or problem you'd like to solve

I need a visual indicator to distinguish when an agent is performing work and when it is waiting on input.

### Proposed solution

The spinning indicator in the title bar currently means two different things, which makes it hard to know whether Copilot needs me or is actively working:

1. The agent is processing / running tools (legitimate spinner)
2. The agent is blocked waiting for me to respond (e.g., ask_user prompt, permission prompt, or just idle after a turn)

Both look identical, so I sometimes wait on Copilot while Copilot is waiting on me.

Request: A 3-state indicator with distinct glyphs:

| State | Suggested glyph |
|---|---|
| Agent working / running tools | 🔄 (current spinner) |
| Awaiting user input (ask_user, permission, idle post-turn) | ⚠️ |
| Turn just completed successfully | ✅ (briefly, then idle) |

Bonus points if the glyphs are configurable via settings.json or a new /theme key, e.g.:

```json
"indicator": {
"working": "🔄",
"awaiting": "⚠️",
"done": "✅"
}
```

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先跟踪标题栏指示器，以及工具执行、用户提示、权限和已完成回合的状态转换。定义并验证彼此不同的工作中、等待输入和短暂完成状态；issue 建议的可配置字形是可选扩展。确认在相关工作流中可以通过视觉区分这三种状态。

由索引模型根据 Issue 内容生成。