feat: Streamlining `cortex run` behavior

[dan-menlo]

Goal

• We will align with Docker on cortex run
  • https://docs.docker.com/engine/containers/run/#foreground-and-background
• We will deprecate cortex chat

Spec

# `cortex run`
# default: runs attached
cortex run <model>  
cortex run <model>  -a --attached                          
cortex run <model>  -d --detached          

For DevOps use cases, we can also recommend

# Chain command (downloads model)
cortex run <model> -d

# Specific model start
cortex model start