feat(settings): setting the default_agent is lacking

[grantcarthew]

It doesn't check the currently available agents and will take any value.
It would be better if the command worked like:

start config settings default_agent
 1. list
 2. of
 3. agents
 choose etc.

and:

start config settings default_agent not-a-real-agent
Warning message
 1. list
 2. of
 3. agents
Choose etc.

and:

The command accepts any value for the default agent without validation