Vision With Capability Detection

Prompt:

Add vision support with nxusKit and verify model capability detection.

Starter fixture: examples/fixtures/starters/go-service/

Expected behavior:

• Consult vision and capability-detection.
• Treat vision as Community Edition when the provider/model supports it.
• Add image input through URL or base64.
• Check model-level capabilities when available, then provider-level capabilities.
• Include a tiny local image fixture or documented image input so smoke testing does not depend on a random external URL.

Verification:

go test ./...
go run . -provider ollama -model llama3.2-vision:latest -vision

Good result:

• The app refuses text-only models clearly.
• A vision-capable model receives an image and returns content.