Add a timer

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

Timer to show how long Agent Thought

### Proposed solution

It would be nice if Copilot CLI showed how long the agent was thinking.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by locating where the Copilot CLI renders the agent's thinking or progress output. Determine how elapsed time should appear during that display, then verify the timer is visible and updates correctly without disrupting normal CLI output.

由索引模型根據 Issue 內容生成。