feat: Add debug logging and --verbose flag (M4.2)

[nathanhuh]

Summary

Add structured debug logging to a file and a --verbose flag for troubleshooting.

Details

• Write debug logs to ~/.config/unic/logs/ with rotation
• Add --verbose / -v global CLI flag to enable verbose output
• Log AWS API calls, auth flow steps, config resolution, and errors
• Structured log format (timestamp, level, component, message)

Checklist

• Add --verbose flag to root command in internal/cli/root.go
• Set up log file writer in ~/.config/unic/logs/unic.log
• Add log rotation (by size or date)
• Instrument AWS API calls with debug logging
• Instrument auth flow with debug logging
• Instrument config resolution with debug logging
• Write tests