Implement cosmiconfig-based configuration system

Description

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

Tasks

• Add cosmiconfig dependency
• Create cosmiconfig implementation
• Update config.ts to use cosmiconfig
• Remove config CLI commands
• Add 'init' command to generate a default config file
• Add tests for the new configuration system
• Update documentation

Benefits

• Modern, standard configuration approach used by many tools
• Ability to use JavaScript for configuration (more flexible)
• Better developer experience with a single config file
• Simpler codebase by removing CLI config commands

Implementation Notes

• The new system will use mycoder.config.js as the primary configuration file
• Configuration can still be overridden by CLI options
• The init command will generate a default configuration file