Add OpenRouter as third image generation backend

[luyao618]

Summary

• Adds OpenRouter as a third image backend (--model openrouter) alongside Gemini and Grok, providing flexible access to any OpenRouter-hosted image model via --openrouter-model <id> (default: openai/gpt-5-image-mini).
• Dynamically estimates per-generation cost by querying the OpenRouter /v1/models API, checking pricing.image, pricing.request, and pricing.prompt/pricing.completion in priority order — so budget tracking stays accurate regardless of how the model is priced.
• Adapts the modalities field per model: requests ["image", "text"] for models that support both, ["image"] for image-only models (FLUX, Seedream, etc.), preventing rejected requests.
• Parses multiple OpenRouter response formats: choices[].message.content[] image_url entries, choices[].message.images[], and DALL-E-style data[].b64_json.
• Caches the model catalog per process to avoid redundant API calls.
• Updates README, setup.md, asset-gen.md, and asset-planner.md with OpenRouter documentation.

Why

Gemini and Grok cover most use cases, but having a third backend is useful when:

• A user doesn't have Google or xAI API keys but has an OpenRouter key
• A specific OpenRouter-hosted model fits the asset better (e.g. a different style or quality)
• Gemini or xAI is down or rate-limited

Changed files

File	Change
skills/godogen/tools/asset_gen.py	OpenRouter backend implementation, dynamic pricing, adaptive modalities
skills/godogen/asset-gen.md	CLI docs for --model openrouter and --openrouter-model
skills/godogen/asset-planner.md	Updated cost table and backend selection guidance
README.md	Added OPENROUTER_API_KEY to prerequisites
setup.md	Added OPENROUTER_API_KEY to API keys section

Test plan

• asset_gen.py image --model openrouter --prompt "red ball" -o test.png with OPENROUTER_API_KEY set — verify image generation works
• --openrouter-model google/gemini-2.5-flash-image — verify custom model selection
• Run without OPENROUTER_API_KEY — verify clear error message
• set_budget 10 then generate with openrouter — verify budget check uses dynamic cost, not flat 5¢
• Verify budget.json log records actual estimated cost

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