Refactor to return errors instead of calling os.Exit() directly:
- Add SilentExitError type for commands that signal status via exit code
- Update mail.go runMailPeek() and runMailCheck() to return errors
- Change Execute() to return int exit code instead of calling os.Exit()
- Move os.Exit() call to main() where it belongs
This improves testability, enables graceful shutdown, and follows Go
conventions for library code.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
637f38edca