Implement new cosmiconfig-based configuration system

Description

Replace the existing CLI config commands with a cosmiconfig-based approach that uses a mycoder.config.js file.

Tasks

• Add cosmiconfig as a dependency (already done)
• Implement config loading using cosmiconfig
• Remove all existing config CLI commands
• Keep CLI override options like --githubMode
• Add an 'init' command to generate a default mycoder.config.js file (already exists, updated with more info)

Benefits

• Simplified configuration management
• Following industry standards for configuration
• Better developer experience