Add a timer

[anaborges02]

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

[MrRishabhJain]

This overlaps heavily with #3278 (per-response elapsed time: live counter during generation + "Done in Xm Ys" on completion). "Add a timer" for thinking time and a per-response elapsed counter are effectively the same feature in the terminal render path — they'd likely be implemented together. Cross-linking so the signal isn't split: #3278.