Support for Session Config Options

[catern]

Support the protocol functionality described in https://agentclientprotocol.com/protocol/session-config-options

Namely, support receiving configOptions during session setup, sending set_config_option requests, and receiving config_option_update notifications.

This is not too important right now since I don't think support for this is very widespread yet.

I specifically want this for my ACP agent for at least two reasons:

• I'd like to be able to configure thought_level separately from model
• config_option_update notifications will allow the agent-shell UI to update when the model is changed by some other client connected to the same agent session

Checklist

• I agree to communicate with the author myself (not AI-generated).
• I've read the README's Filing issues section.
• I'm running the latest versions (fill in below).
  • agent-shell version:
  • acp.el version:
  • ACP package (e.g. claude-agent-acp) version:
  • Agent CLI (e.g. claude, gemini) version:
• [] For requesting new agent support, I'm including a link to the ACP-capable agent or related ACP package.
• [] For issues, I'm including ACP traffic (as per README).