fix: stop changing main model/agent from subtasks invocation

[dbpolito]

What does this PR do?

Currently /review, even having in the prompt to not make changes, very often it decides to go ahead and do the changes.

This PR changes to use plan agent.

Also, i noticed that currently even with subtasks, when you run a command with a different agent, it's changing the main session agent to that, this PR also fixes it.

How did you verify your code works?