Resume profile sessions in the profile's model and effort

Summary

• When fx starts in an agent profile, or /launch <profile> opens an instance, the instance's first session saves the profile's model and effort, so fx --resume, fx -c, and fx -r use them again instead of your configured model and effort.
• Resuming a group of instances brings each instance back in its own profile's model and effort.
• A session started later with /new keeps the model and effort the instance uses by then, as before.
• --model, --effort, --provider, and FX_MODEL still apply only to the run that passes them, and the session doesn't save them.