feat: extract models from command-code dist, load via models.json

- Add scripts/extract-models.ts to parse command-code npm dist file
- Generate models.json (21 models, 15 pricing entries) with contextWindow
  and maxOutputTokens pre-filled; no nulls or hardcoded fallbacks in index.ts
- Rewrite index.ts to load model list and costs from models.json
- Cap gateway model maxOutputTokens at 65536 (API limit for Baseten/Vercel)
- Add 'Update models' section to README documenting the generation flow
- Add npm run extract-models script