fix(image): add --out flag for saving to exact file path

[RyanLee-Dev]

Summary

• Add --out <path> flag to image generate for saving to an exact file path
• Validates that --out cannot be used with --n > 1 (use --out-dir instead)
• Auto-creates parent directories if they don't exist
• Consistent with speech synthesize --out behavior

Test

# Save to exact path
mmx image generate --prompt "A cat" --out /tmp/cat.jpg
# saved: /tmp/cat.jpg

# Error when used with multiple images
mmx image generate --prompt "A cat" --n 3 --out /tmp/cat.jpg
# error: --out cannot be used with --n > 1. Use --out-dir instead.

Closes #112