Refactor command handlers into separate files to improve code organization
and maintainability. Each command (agent, auth, cron, gateway, migrate,
onboard, skills, status) now has its own dedicated file.
Restructure provider creation to support new model_list configuration
system that enables zero-code addition of OpenAI-compatible providers.
Move legacy provider logic to separate file for backward compatibility.
Move configuration functions from config.go to separate files
(defaults.go, migration.go) for better organization.
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