Replace docopt with optparse-applicative

Docopt has issues with option ordering (options must come after positional args). Switch to optparse-applicative for proper CLI parsing.

Scope:

• Omni/Cli.hs - core CLI framework
• Omni/Agent.hs
• Omni/Agentd.hs
• Any other modules using Cli.docopt

Benefits:

• Options work in any position
• Better type safety
• Auto-generated help
• Widely used/maintained

Reference: optparse-applicative on Hackage