Improve auto branch naming ergonomics

[snoolord]

Summary

• allow workmux add to accept unquoted multi-word branch input and normalize it to kebab-case for ergonomic branch creation
• add auto_name.command so --auto-name can invoke agent-specific CLIs like opencode run or claude -p instead of requiring llm
• add regression tests covering CLI normalization, custom auto-name command execution, and config parsing for the new field

Test Plan

• cargo test llm::tests
• cargo test cli::tests
• cargo test parse_auto_name_command

[raine]

Thanks! I've included the auto_name.command idea in slightly different form. Now in main. Still thinking about the workmux add var args thing.