copilot --resume does not remember previously used agent

[soungsid]

Description:
When using copilot --resume, Copilot correctly lists previous sessions.
However, after selecting a session, the agent used in that session is not remembered, forcing the user to manually reselect the agent.

In practice, it’s easy to forget to reselect the correct agent.
This can lead to:

• Loss of context or reduced understanding of the ongoing session
• Potentially unnecessary credit consumption due to the wrong agent being used

Automatically restoring the previously used agent when resuming a session would greatly improve usability and continuity.

Expected behavior:

• Resuming a session should automatically restore the agent that was used when the session was created.

Actual behavior:

• The agent must be manually reselected every time a session is resumed.

Motivation:
This would improve continuity and reduce friction when working with long-running or iterative Copilot CLI sessions.