Add nano_banana image generator adapter

[adelaidasofia]

The nano_banana adapter is stubbed in image_generators/nano_banana.py but not implemented. Contributions welcome.

What's needed:

• Implement NanoBananaAdapter.generate(spec) returning {status, adapter, image_path, ...}
• Honor the style_prompt and output_dir fields from config.json under image_generator.nano_banana
• Download the result to spec["output_path"] as PNG (1080x1080 preferred)
• Follow the ImageGenerator ABC in image_generators/base.py

Spec the adapter receives:

{
  "pillar": "P1" | "P2" | "P3",
  "quote": str,
  "handle": str,
  "figure_path": str | None,
  "output_path": str,
}

See image_generators/pillow_local.py for a complete reference implementation.