Introduce experimental session modes

[agu-z]

Enables the agent to expose a set of modes that the session can be put in at any given point. The main motivation for this is enabling a permission mode selector for Claude Code, allowing you to toggle between "default" and "plan" mode.

Note: This feature is experimental, and may be removed or changed at any point

[ConradIrwin]

@agu-z Instead of making this stateful, I think it'd be cleaner to just add a mode: Option<mode> to session/prompt. (Assuming that you can't meaningfully changes this while the model is generating output).

[agu-z]

@ConradIrwin yeah, that's a good idea!

That said, since making this PR I realized that a core feature of plan mode in CC is that it can ask the user to switch modes in the middle of a turn when it thinks it's ready for implementation. So it's not quite stateless.

I'm gonna try a few approaches for this mode switch on Monday. I think the simplest would be to request the switch via a tool call, but we'd also need a notification from the agent to let the client know that the mode changed. Another option is to add an explicit method to request a mode switch, in which case the notification wouldn't be needed.