Follow ADDINGMODEL.MD and add gemini-3.5-flash

[juanmichelini]

See details https://ai.google.dev/gemini-api/docs/models/gemini-3.5-flash

Remember to test it with gh and the correct ref.

[juanmichelini]

@OpenHands

[openhands-ai]

I'm on it! juanmichelini can track my progress at all-hands.dev

[openhands-ai]

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