Enhancement: Agent Switching should remember selection per repo

[rcdailey]

I have different agent configurations in various repos I work out of. These agents are not shared across repos. However, plannotator remembers the same agent name no matter what repo I work out of, so I'm constantly switching it.

• The agent selection box should not be in settings, it should be in the forefront: Make it a selection box right next to the approve button.
• This would mean the "warning" modal when the selected agent doesn't exist would go away -- instead, users cannot click submit while an invalid agent is selected.
• And of course, the agent selection should be remembered uniquely for each repo.

[backnotprop]

good idea @rcdailey