feat(node): support input-guided image generate intent

[kvz]

Summary

• let image generate accept repeated --input values while keeping prompt-only text-to-image working
• bundle provided inputs into a single /image/generate assembly so prompts can refer to files by name
• restore local prompt and --num-outputs validation, with regression coverage

Testing

• yarn check
• ~/code/dotfiles/bin/council.ts review
• yarn vitest run packages/node/test/unit/cli/intents.test.ts